Its still a pretty fragile
After going through theSharepoint/ Silverlight BluePrints, you still have to do a whole heap of work to get something working. See
Patrick Tisseghems Blog [ http://www.u2u.info/Blogs/Patrick/Lists/Categories/Category.aspx?Name=Silverlight%20BluePrint ],
For the most robust advice, unfortunately a lot of web.config copying is involved.
I have had some partial success, and can develop Silverlight 2.0 XAP files displayed in SharePoint Webparts, but I am having Authentication problems for Member users to see the WebParts, unless the Site Administrator logs in first