Hang After Update
I've got the same issue as Dillinger on my MacBook. Ran update, left it alone for a while to d/l and install, came back sometime later with a message saying one of the updates didn't install, clicked ok and it went about restarting, but never did, it just hung on my wallpaper with no finder/icons/menu bar. I waited for at least 5 minutes if not more, then hit the power button, turned it back on and it just hung on the gray Apple screen with the swirling circle.
I left it alone like that for 15 or 20 minutes before trying to power down and up again, still same hang. Popped in OS X disk 1 and ran Disk Repair, it repaired something (volume header I think, I've been at this all day now and my head's gone a bit buggy). Restart and same hang still.
Disk Repair notes that OS X is version 10.4.9, so it looks like that update was installed. Trying Repair again now says nothing to repair.
Checked Apple support docs online and tried the Safe Mode boot, gave it another 10 or 15 minutes on the Apple screen (it said could take 2 or 3 minutes to boot in Safe Mode) still nothing.
Now I've managed to get my hands on an external drive and am in Disk Utility making an image of my Mac HDD in the hopes that I'll be able to go through it and pull out files in case the drive itself needs reformatting/reinstalling of OS X. Maybe you can't do that with a disk image, I don't know, but I'm willing to try anything at this stage. May try Dillinger's discovery of being able to install OS X over again without affecting files, though the thought does make me a bit nervous.
Regarding backing up, lesson learned! I've got lots of stuff backed up to DVD-Rs but there's still a lot of recent files and documents that are hanging in the balance on the HDD. Time Machine, you're late! How ironic.
Will be calling Apple once my image is done (its taking a while, 53+GB of data) and seeing if my 90 days are worth anything in getting this fixed. I'll be sharing any findings here with you guys; hopefully if I get this fixed the same thing will work for the rest of you in the same boat.