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.