actions/ajaxTester/ajaxTester.php

<?php

$strInput = get_input('testInput');

system_message("your input: ". $strInput);

forward(REFERRER);

?>

Just wanted a give this a little bump to see if there are any ideas on how to solve this issue. 

you're using jqm;s pageinit() api. i don;t think you'll find many people here @elgg comm who have worked with that already or people wh have time to dive in to learn something.. so maybe that's why u r getting no 'answers'. but try explaining more details of the "'undefined' back from the action..." in elgg context and maybe someone will know what's going wrong.. ;-)