Two recent Apple KB articles of note: 106605: Mac OS X 10.1: Do Not Reinstall After Using the Mac OS X Update 10.1.1 states: "You should not reinstall Mac OS X 10.1 ...

We assume this means that when a 10.1.1 update (or whatever the next update will be called) comes out, you will not be able to install the update unless you have previously installed this Security ...