You can build SharePoint Web Parts to take advantage of Silverlight user interface components. You can find out how in the Silverlight Blueprint.
 
The Silverlight Blueprint for SharePoint is source code and guidance for developers describing how to use Microsoft Silverlight and Microsoft SharePoint in your applications and Internet Web sites. SharePoint applications that use Web Parts can now be built using Silverlight user interface elements. This capability enables a fresh look at data exposed through SharePoint Products and Technologies using the modern graphics capability in Silverlight.
 
It’s important to note that this is not a new supported Microsoft product and doesn’t necessarily represent future product direction it works with Windows SharePoint Services 3.0 and Silverlight 2.0. But we think this will help you build the latest generation of SharePoint sites. Athought there is sample code for you to use of Silverlight with SharePoint, there is guidance for you integrated into Visual Studio.
 
For more information, see the MSDN article, Silverlight Blueprint for SharePoint. The article includes a link to the downloads, including screencasts and examples of Web Parts build with Silverlight. There is also a Channel 9 video on Silverlight Blueprint for SharePoint and a few other cool resources if you check out the link.