Thinking of messing with Longhorn?
Be sure to go here first: Longhorn Developer FAQ
You'll discover all kinds of helpful information that you will need to know up front. For instance, if you're just now getting into the Longhorn game, and you are thinking that you'll install the WinHEC build of Longhorn with the latest May 2004 Community Drop of Whidbey on top of it, guess what? Wrong! The WinHEC build of Longhorn does have plenty that's been added since PDC, BUT, there is no build of Whidbey that will run on it. In fact, you might just hose your WinHEC Longhorn if you put Whidbey on there.
So if you want the Longhorn/Whidbey 1-2 punch, stick to the PDC builds. The FAQ will tell you how to get them.
-Eric.