The sidebar is gone. Sidebar gadgets are now free-floating entities on the desktop. Much nicer way of doing things.
Under vista you can also use the gardgets free-floating.
That said, to me the solid-bar-on-the-side-of-a-widescreen-monitor seems a much neater setup that a ton of free-floating stuff, so I sure hope that you can go back to the old behavious as well.
@VinylPusher: Thanks for the info. Gah I guess this means my games will need to support it when it comes out, unless it works the same as Vista internally.
Most likely it's pretty much the same as Vista for any of the major components: Ironically, Windows 7's internal version number is 6.1 (with Vista being 6.0)