Microsoft does support hosting the silverlight plugin in a variety of different scenarios, see: http://msdn.microsoft.com/en-us/library/dd550717%28VS.95%29.aspx#using%5Fsilverlight%5Fin%5Fa%5Fbrowser%5Fcontrol

Can you explain more about your situation? Is the WPF application an XBAP, or a non browser hosted application. Would you want to publish the silverlight portion of the content on a website, or try to embed it with the application? The story is a bit trickier if you want complex communication between the silverlight content and the rest of the content being displayed and vice versa.

-Graham