Steve Riley had a good long post on his blog about Mandatory Integrity Control as it is implemented in Vista that drew even longer comments.

Great concept, as you will see from several of the comments, this isn’t the first implementation, but I expect it will be the first to get nearly universal distribution

The big concern is whether the bugs will be worked out for release.  I am betting yes, though I expect a Service Pack will come someday to bring the real value of this home.