To elaborate on some of what Cash said:

• A beta for 1.8 is scheduled for the middle/end of autumn.  This can change.
• For non-theme plugins, the biggest change will be the layouts used in elgg_view_layout().  So far it's as simple as as changing a string and swapping the order of the arguments passed.
• I'm considering writing a compatibility wrapper "theme" for 1.7 plugins whose authors choose not to upgrade them.  Since non-theme plugins are generally easy to update and this will take significant time away from actually developing and testing 1.8, it might not be worth it.
• Don't use 1.8 for production.  Don't use anything in SVN for production.

Thanks guys!

THX to Brett and Cash ... you guys doing a good job .. and hope you will push elgg in a new sphere with 1.8 ... if you need some ideas ask me or check my featurerefquests in this forum ;-)