The Silverlight 1.1 alpha bits don't currently have much in the way of controls. The infrastructure is there (you can derive from System.Windows.Control, for instance), and we include a few early sample controls (button, slider and so on, including source code) with the SDK, but the full set of controls won't come until the next public release.
In the meantime, Tim Heuer pointed me at an interesting set of controls that have been developed by an organization called NETiKA Technologies. If you go to this demo page, you'll see that they've somehow taken a pretty broad set of Windows Forms controls and implemented them for Silverlight (and Flash). Or at least, I think that's what they've done. It's not entirely clear what's going on under the covers here, since the code you write starts off as a Windows Forms application that the toolkit cross-compiles to Silverlight.
The demos are pretty impressive - a little slow (it is alpha code running on alpha code, to be fair), but particularly the designer demo is a great example of the RIA experiences you'll be able to build and run on Silverlight. Another couple of weeks' work, and this demo could be a real web-based Silverlight IDE in its own right!