Mike Gunderloy commented today on the recent Microsoft beta programs:

Boy howdy, there are a lot of people talking about the Whidbey (Visual Studio .NET 2004) and Yukon (SQL Server 2004) betas. I'm sure I've ranted about this before, but remember, only two types of people brag about being in Microsoft beta programs: liars and cheats. Well, OK, maybe there's a third category: careless people. Read the NDAs you signed, people. From time immemorial, Microsoft beta program NDAs have prohibited discussing the beta program, or even the fact that you are involved in the beta program. I'm as excited about coming software as the next guy, but the general blabbermouthness of beta testers is the reason why some MS programs are moving to much tighter, private betas with a select group of people who know how the heck to keep their mouths shut.
I tend to agree with Mike on this one.  While it is fun to hear about the new stuff and hear what other people think, if you're not in the program you're not supposed to know about it.  If you are in the program, you're supposed to keep it confidential.  I wish the programs could be more open, but that's just not the world we live in.  Eventually all of the programs go public, and that's when everyone can start talking.  Those lead times are pretty long, so I don't think the process suffers at all as a result of the private beta. 
I do take exception with the lumping of Whidbey and Yukon into the same bucket though- I think the Yukon beta stuff has been much more quiet.  But that's just the SQL developer in me talking.