One day I'll notice an update and post this a bit earlier, but...
For anyone having trouble with the well-known
"OMG, (insert update name here) hosed my Mac" syndrome, I have a question. Do you (a) just let your Mac sit there while it downloads and runs the update, or do you (b) carry on working?
If you answered (b) then you need to visit
Unsanity's blog, here and read their analysis.
Definitely worth a read. I had long ago developed a "superstitious" fear of doing anything else while my mac updates itself but this article justifies and explains that fear. Apparently, there's a 100% reproducible bug on the prebinding (optimisation) that can render your mac unbootable. It's OK to carry on working while your Mac downloads an update and it's OK to carry on working while your installs it but, while optimisation is running, do not touch anything.
Effectively, this means that it's best to go and make a cup of tea while software update works its magic.