10.6.8 has bricked my Mac Pro. I have repaired disk permissions, verified the disk, and reinstalled OS X from DVD, then installed the combo update. About the only thing I haven't done is clean off the disk and perform a bare metal install, but frankly a dot release isn't worth that sort of hassle. So for the moment I have reverted to 10.6.7 and I'll wait for either some sort of mea culpa from Apple, or Lion, whichever comes first.
FWIW, 10.6.8 installed without issue on my MacBook Pro, and it's running pretty much the same collection of software as the Mac Pro.