Our friend Paul Thurrott has a report that PDC attendees won't be getting a copy of Aero. They will get a copy of the Longhorn alpha but the Aero technology will be removed from it.
Instead, they'll get a demo of it at the show. What is Aero? It's Microsoft's "next generation" concept of what the Windows GUI should be like. They have (ahem) concerns that others might "steal" their ideas (such as Apple).
Of course, as we've all seen, Mac users believe Apple's invented everything. Alpha blended shadows? Mac first right (no, WindowFX first). Zooming icons first? (no, DesktopX first). And so on. And now it appears that the new Expose tech in Jaguar came first from Microsoft.
But don't feel too sorry for Microsoft, I think they've been a net benefitter of borrowed ideas over the years. 
