I am happy to announce that I have uploaded a new project called SilverMap to CodePlex. SilverMap is a tile map control for Silverlight games that uses layered maps where the higher layers are drawn over the lower ones. In addition, individual tiles have a z-index position and can be drawn over one another. You can also place the tiles anywhere on the map, instead of in a tight grid, which is both beneficial and kind of a pain. :)
SilverMap makes use of the WriteableBitmapEx library. I also used Danc's Miraculously Flexible Game Prototyping Tiles while testing and included them as a zip with the project.
The maps that are created can be saved as XML and the layering information is stored with them. You can set the opacity of a tile and in the near future will be able to scale and rotate them. Aside from the editor the SilverMap.UIMap control can be included in your game project and has the ability to load maps from a file stream (useful for maps that download on the fly).
The code is freely available under the Microsoft Public License so feel free to use it your games, whether they are free games or not.