I saw a Hansleman tweet about his investigating the Mesh dll's and that the CoreCLR.dll was in there. Indicating that a desktop app might be able to use a reduced CLR via the Silverlight DLL. One of the first comments from that pointed to this http://www.blendables.com/labs/Desklighter/Default.aspx which looks like it has used the Silverlight CLR for a desktop app.
There was something that I saw about someone might learn Silverlight and then approach desktop apps using WPF before doing it the other way round. There may be something to that idea. I will have to investigate that more.
Hansleman indicated that this could be a way of doing some cross platform development with a much smaller foot print that the whole .Net Framework.
It makes me wonder about how the licensing reads for using Silverlight - can you use it offline?