It seems we can't keep things in one place for more than a couple of months. Back in January I posted links out to the aforementioned spreadsheets. We have moved them and made a few updates which you can find below. I also rounded up a couple of other interesting references for Group Policy for your geek reading pleasure...
For Windows Server
Changes in Functionality from Windows 2003 SP1 to Windows Server 2008 - Very Good Read!
For Windows Vista
Windows Group Policy Home Page on TechNet - This is THE source for all things Group Policy
That should keep you busy for a while!