last known good configuration

Without manually making a backup, it's not currently possible. When Mac OS X 10.5 (Leopard) comes out "this spring", it will include an incremental backup option called "Time Machine" that will allow you to restore files and folders to a previous configuration.

-Doug

The best you can do now is to maintain a clone on an external drive with a program like SuperDuper. If an update goes bad, or something just plain doesn't work you can restore from that clone.

I have been doing this for about 2 years and have yet to need to restore. I am amazed at the stability of OS X in comparison to Windows. I am not a Windows-bashing Mac zealot, but I have had more issues with my PC than my Mac on those same 2 years.