So I finally came around to upgrade to PHP 5.2 (I was running 4.4 on the production server until now). I know, what a shame! :)
Anyway the only issue I really had, in spite testing the code really well over time, was with class_exists() and millions of warnings becaus of a missing __autoload(). I blame the people who introduced the second parameter to class_exists() and changing the default behaviour at the very same moment. Actually, it was an annoying but rather easy to fix compatibility brake.
APC in conjunction with PHP-5.2 seems to work very well either, it even feels a lot less memory exhaustive than running APC+PHP-4.
Ah yes, just on a side note, I also upgraded to Apache-2.2 in the same run. Somehow I now have a warm and fuzzy feeling running this up-to-date software, it really was already like a brinck in my stomach. Anyway, what's left is to migrate from the Apache module to the FastCGI SAPI and switching to the worker MPM... but I'm done already.