I was unable to login because /usr/bin/X11/sessreg, which gdm's default PreSession script invoked, did not exist. I removed some of the old X stuff and installed some new things (xutils was the key, I think) and got the file.
I also had the usual X tweaking to do with the new setup. Although it's not documented, XF86 4 has an "IntelliMouse" option to the "mouse" driver, and that's what worked for me (3 button Logitech PS/2 mouse). None of the "XXXPS2" settings worked.
