Just in case anyone is wondering...

I did the upgrade to 10.6.4 and got a kernel panic. It seems like it's related to the SleepEnabler kext. I believe there were problems of this same nature going from 10.6.2 to 10.6.3. I haven't been able to resolve it yet since my Mac OS installation will not load, even in safe mode. I think it's safe to say that people may want to remove the kext before doing the update.