A tiny feature request, but it makes the IDE oh so more complete.

In the auto complete popup for classes (when you type 'new ...') it would be helpful to distinguish between normal classes and abstract classes (which you can't instantiate using 'new') by using a different icon or formatting (how about gray + italic?).

Agreed, except I don't think abstract classes should be shown at all.

I also tested adding the @abstract tag to the class's phpdoc comment, which you might do if using PHP4.
Adding the tag did not affect the autocomplete popup either. If the @abstract tag is set, I think the class should be hidden from autocomplete as well.

I would prefer JeRa's suggestion.

Just because the class is abstract doesn't mean it should be, might be a mistake on someone elses part, by not appearing in the popup how would you even know if existed?

Light grey would work with a new icon.