Although I've seen reports by others of problems with major Apple updates, I've never had a problem . . . until today.
Instead of using Software Update, I actually downloaded the updater from Apple's site -- took about a hour on a T1. I ran the updater, restarted, and my computer went into an infinitely looping blue screen on my Cinema Display.
Since I figured I was screwed regardless, I opened up my laptop, held down the power button to force a power down, hit the power switch to power up, and ran an errand for about 15 minutes. When I returned, I was presented with the login screen, but my Cinema Display wasn't displaying a picture. I logged in, ran a disk permission repair, restarted, and everything was good.
Now that I'm back up, I did a little Googling and it appears that my problem is not uncommon, so I would suggest doing a little searching yourself and arming yourself with some potential troubleshooting solutions before you run the update.