One of my curret side projects is a nifty online game using both Silverlight and the Facebook API. Since this is primarily a learning experience, I figure I would share some of the more annoying issues that I ran accross, and some solutions.
The first issue I ran ito is that my Silverlight app would completely overlap the facebook status bar. All efforts at google-fu failed me, but eventually through a convoluted series of posts and searches came down to the issue and the solution.
If you plan on hosting a Silverlight usercontrol in a Facebook iFrame, there are two properties you will want to set. First, the max width (to avoid scrollbars and whatnot) is 758 pixels. Second, to avoid overlapping your Facebook status bar, you will want to turn Windowless mode by adding Windowless=true to your Silverlight tag in your host ASPX page.
Hope this helps someone out there!