Thanks a lot for your help! I found that lsws/fcgi-bin was only writable to root. Opening it to litespeed did the trick. So it seems that the core dump location moved at some point from the php script dir to the lsphp5 dir.
And from the core dumps, it seems that they're occuring during shutdown and some other points when linking libraries - no php user functions in the backtrace. I'm on 64-bit CentOS 5. I noticed that at some point I must have lost "--with-libdir=lib64" from my php configure, so I pretty sure that's the problem. Thanks again.