It would be very helpful to recompile the geo types so they can be used in Silverlight. Right now the workaround is to convert the geo types to a path geo string on the server and then send to Silverlight and decode.
One obstacle is that the Microsoft.SqlServer.Types assembly uses a native DLL to do its heavy spatial computations. This would seem to prevent a Silverlight-compatible implementation, since Silverlight is cross-platform and only supports managed code.
But, as Tanoshimi pointed out, you can create a new feature request in Connect, and if enough people agree that it's valuable, then it may pursuade MSFT to take action.