We do not have any official information on when the support for Silverlight will happen, but it is definitely something that will come the near future.
This website is always updated with the latest information and news. I would recommend you to keep and eye on our forums and also on this link for latest information in this topic: http://appdeveloper.intel.com/en-us/develop
Please do not hesitate to contact us if you have any other queries.
Intel Technical Support
Intel® Atom™ Developer Program
Intel AppUp(SM) Center
To add to Roovens details: It was mentioned at IDF that the .NET version of the AppUp SDK was coming soon as well. One could easily create their own Silverlight Wrapper Assembly now if they wished. If you need any assistance creating your own in the meantime please let me know.