This has been discussed numerous times before but Windows Vista brings it up once again.
I went on a rant on this on this week's FoxShow related to user interface design but I agree.
"As I pointed out last week, it's not a beta until it's feature-complete. Microsoft is showing off a prototype, a demo. When they have the new UI in place, and lock down the features they plan to actually ship, then it is time for beta evaluation and testing."
Ted's Radio Weblog