The only example I have is for a client and it is a little complex as they had a fancy form. But here you are:

<p>{@on_plugin:registration:introduction}</p>
<div id="registration_section1">
    <h3>{@on_plugin:registration:header1}</h3>
    <p>{@on_plugin:registration:description1}</p>
    <div class="registration_item">
        <div class="registration_item_label">
            {$voorletters:t}
        </div> <!-- registration_item_label -->
        <div class="registration_item_input short">
            {$voorletters:i}
        </div> <!-- registration_item_input -->
        <div class="registration_item_description appended">
            {$voorletters:d}
        </div> <!-- registration_item_description -->
    </div> <!-- registration_item_item -->
    <div class="registration_item">
        <div class="registration_item_label">
            {$voornaam:t}
        </div> <!-- registration_item_label -->
        <div class="registration_item_input">
            {$voornaam:i}
        </div> <!-- registration_item_input -->
    </div> <!-- registration_item_item -->
    <div class="registration_item">
        <div class="registration_item_label">
            {$tussenvoegsel:t}
        </div> <!-- registration_item_label -->
        <div class="registration_item_input short">
            {$tussenvoegsel:i}
        </div> <!-- registration_item_input -->
    </div> <!-- registration_item_item -->
    <div class="registration_item">
        <div class="registration_item_label">
            {$achternaam:t}
        </div> <!-- registration_item_label -->
        <div class="registration_item_input">
            {$achternaam:i}
        </div> <!-- registration_item_input -->
    </div> <!-- registration_item_item -->
    <div class="registration_item">
        <div class="registration_item_label">
            {$geboortedatum:t}
        </div> <!-- registration_item_label -->
        <div class="registration_item_input medium">
            {$geboortedatum:i}
        </div> <!-- registration_item_input -->
        <div class="registration_item_description appended">
            {$geboortedatum:d}
        </div> <!-- registration_item_description -->
    </div> <!-- registration_item_item -->
    <div class="registration_item">
        <div class="registration_item_label">
            {$geslacht:t}
        </div> <!-- registration_item_label -->
        <div class="registration_item_input">
            {$geslacht:i}
        </div> <!-- registration_item_input -->
    </div> <!-- registration_item_item -->
</div> <!-- registration_item_section1 -->
<div id="registration_section2">
    <h3>{@on_plugin:registration:header2}</h3>
    <div class="registration_item">
        <div class="registration_item_label">
            {$_name:t}
        </div> <!-- registration_item_label -->
        <div class="registration_item_input">
            {$_name:i}
        </div> <!-- registration_item_input -->
        <div class="registration_item_description after">
            {@on_plugin:registration:name:description}
        </div> <!-- registration_item_description -->
    </div> <!-- registration_item_item -->
    <div class="registration_item">
        <div class="registration_item_label">
            {$_password:t}
        </div> <!-- registration_item_label -->
        <div class="registration_item_input">
            {$_password:i}
        </div> <!-- registration_item_input -->
    </div> <!-- registration_item_item -->
    <div class="registration_item">
        <div class="registration_item_label">
            {$_password2:t}
        </div> <!-- registration_item_label -->
        <div class="registration_item_input">
            {$_password2:i}
        </div> <!-- registration_item_input -->
    </div> <!-- registration_item_item -->
</div> <!-- registration_item_section2 -->
<div id="registration_section3">
    <h3>{@on_plugin:registration:header3}</h3>
    <p>{@on_plugin:registration:description3}</p>
    <div class="registration_item">
        <div class="registration_item_label">
            {$adres:t}
        </div> <!-- registration_item_label -->
        <div class="registration_item_input">
            {$adres:i}
        </div> <!-- registration_item_input -->
    </div> <!-- registration_item_item -->
    <div class="registration_item">
        <div class="registration_item_label">
            {@on_plugin:registration:huisnr_toevoeging}
        </div> <!-- registration_item_label -->
        <div class="registration_item_input short">
            {$huisnummer:i} {$huisnr_toevoeging:i}
        </div> <!-- registration_item_input -->
    </div> <!-- registration_item_item -->
    <div class="registration_item">
        <div class="registration_item_label">
            {$postcode:t}
        </div> <!-- registration_item_label -->
        <div class="registration_item_input">
            {$postcode:i}
        </div> <!-- registration_item_input -->
    </div> <!-- registration_item_item -->
    <div class="registration_item">
        <div class="registration_item_label">
            {$woonplaats:t}
        </div> <!-- registration_item_label -->
        <div class="registration_item_input">
            {$woonplaats:i}
        </div> <!-- registration_item_input -->
    </div> <!-- registration_item_item -->
    <div class="registration_item">
        <div class="registration_item_label">
            {$telefoonnummer:t}
        </div> <!-- registration_item_label -->
        <div class="registration_item_input medium">
            {$telefoonnummer:i}
        </div> <!-- registration_item_input -->
        <div class="registration_item_description after">
            {$telefoonnummer:d}
        </div> <!-- registration_item_description -->
    </div> <!-- registration_item_item -->
    <div class="registration_item">
        <div class="registration_item_label">
            {$_email:t}
        </div> <!-- registration_item_label -->
        <div class="registration_item_input">
            {$_email:i}
        </div> <!-- registration_item_input -->
    </div> <!-- registration_item_item -->
</div> <!-- registration_item_section3 -->
<div id="registration_section4">
    <div class="registration_item">
        <div class="registration_item_label">
            {$doenersnet:t}
        </div> <!-- registration_item_label -->
        <div class="registration_item_input">
            {$doenersnet:i}
        </div> <!-- registration_item_input -->
    </div> <!-- registration_item_item -->
</div> <!-- registration_item_section4 -->
<div id="registration_section5">
<div class="registration_item">
        <div class="registration_item_label">
            {$gebruiksvoorwaarden:t}
        </div> <!-- registration_item_label -->
        <div class="registration_item_input">
            {$gebruiksvoorwaarden:i}
        </div> <!-- registration_item_input -->
        <div class="registration_item_description appended-dark">
            {@on_plugin:registration:gebruiksvoorwaarden_text} <a class="gebruiksvoorwaarden_popup">{@on_plugin:registration:gebruiksvoorwaarden}</a>
        </div> <!-- registration_item_description -->
    </div> <!-- registration_item_item -->
</div> <!-- registration_item_section5 -->

By the way, it appears that the standard Elgg captcha plugin redirects to the front page if the wrong answer is given and loses all the registration information. So I wouldn't use that.

At some point I will probably create a captcha field for the form plugins that handles validation properly and redisplays the form.

Thanks Kevin, where should the styling (css) be placed ? 

Can we add the css from the template field ?

You can.

My clients usually create a full blown theme plugin and so we place the registration form CSS there along with everything else.

I have turned input filtering off for the form template fields as they are only used by site admins so you should be able to put anything in there you like.

Great. Perfecto. Thanks again.

I'm also having this problem on WAMP.
In the apache_error log i find: ID for field Adresgegevens is 0, ....

This means that get_entity in manage_form.php is not returning anything ($form = get_entity($form_id);)

Any ideas?

Nope, as stated above, so far as I know this is a server related problem affecting a small number of people and the only known solution currently is to move to another server.

I assume that you are using only ASCII characters in your internal field names, correct?

If you have any new insights, please share them here.

Solved (at least this did it for me): I altered the following setting in the php.ini file and now it works on my WAMP testing environment :-)

output_buffering = 4096
(default it was: output_buffering = on)

Kevin, one more thing. The error i mentioned earlier (comes from form/models/model.php) is not really an error if the field is new ( so in that case it shouln't be logged ).

Ruben, do you have any theories for why making this change to output_buffering has solved your problem?

A quick google finds this suggestion:

http://jlcoady.net/tips/tip-turn-php-outputbuffering-elgg

to turn output buffering off all together when using Elgg.