You are in full control of what goes into the maptip on the graphicslayer, so you can put whatever text you like. That's the simplest approach. See http://resources.esri.com/help/9.3/arcgisserver/apis/silverlight/samples/start.htm#GraphicsMapTip for an example. If you want to use the MapTip control (there's not really much point anymore), you can override the MapTip template and again put whatever you would like to see in the maptip. In Expression Blend, right-click the maptip control, select to edit a copy of the template, and you can replace the datagrid with your own set of textblocks that binds to the attributes, similar to the GraphicsLayer's maptip.
