Sorry to bring up an old thread. I had this same problem also and just solved it. To clarify, my problem was I had a feature layer with a unique value renderer and the only symbol being displayed was the default symbol.
Looking at the rest service details on a browser, I saw all my fields were uppercase and under "unique value renderer" my "field 1" was lowercase.
What I did was open the related map, removed the layer, added the layer back in and rebuild the symbology from scratch, saved, restarted my map service, cleared my rest cache and voila! My unique value renderer - field 1 is now uppercase and my feature layer displays correctly. So it appears in ArcMap renderer fields are not case sensitive (I don't know how it got to be lowercase to begin with), but in Silverlight they ARE case sensitive.
Hope this helps people who come across the same problem.