In a test map, with nothing but an ArcGISTiledMapServiceLayer in it, the CPU usage meter shoots up to nearly 100% (on a dual core machine) when the extents change and the tiles are downloaded. once the tiles are downloaded, the CPU drops back down to 0. i notice that this spike doesn't occur in the sample app http://resources.esri.com/help/9.3/arcgisserver/apis/silverlight/samples/start.htm#Map
the culprit seems to be any containers on top of the map that have an opacity below 1. i'll call this resolved, and a silverlight issue not an ESRI API one.