I had the same problem with ESRI--we were trying to print a map. You can't access the pixels in the image because of cross-site security. If you are hosting the silverlight app on the same server as the ESRI ARCServer, you should have no problem.
There is a printing example on the ESRI samples web site and it works like a charm, but it is because the app and the images are on the same server.
What I ended up doing was totally haxor, but it worked. I used an ashx handler like a proxie to get the base map image that I needed, then drew on top of that image.