I just upgraded my 4-user multiseat system to Fedora Core 6 from FC4+X.org 6.9. The installation went fairly smoothly (though Anaconda fought with my [admittedly odd] RAID configuration and wanted to install the Grub bootloader code on /dev/md1; I switched to vt2 and used the Grub shell to set things right before rebooting).
One thing I wanted to try was AIGLX/Composite/Compiz. I thought that the OpenGL drivers for the cards might get tangled up, but that is not the case -- it actually works well, but the performance hit is heavy. It looks like a dual-core system and/or the upcoming AIGLX optimizations will be necessary to make it really usable.
It's also worth noting that X.org 7.1 seems to work better than 6.9 for multiseat operation; for example, the Zap key combination (Ctrl-Alt-Backspace) works fine on all seats. The old xorg.conf file needed only minor editing to take into account path and module changes and to add the Nvidia closed-source driver's AddARGBGLXVisuals option.