I took a look at my suexec.log:
[2010-03-04 19:33:10]: uid: (5005/web4) gid: (5008/5008) cmd: .php-fcgi-starter
The thing that caught my eye is the "mismatch" of the uid. (5005 and web4 are the same user)
Could that be the problem? And if so, where does it stem from?