Sheesh... OK, I wish I had an answer, but this has gotta be something outside of ubb on the OS (CentOS). I just did a complete refresh of my test system (running Solaris w/ IM 6.3.4) and the test system's captcha worked fine. I'm assuming the differences in version of ImageMagick have nothing to do with it... right?
I did notice that I have a LOT more .ini files being scanned by php. My production box does not have any additional directories to be scanned.
Are the .ini files the place to start looking?