Silverlight-winrt is a project mainly written in C#, it's free.
Replaying the experiment getting the SL2 sample app running on WinRT
There was a demo at the BUILD conference (first day Keynote) which showed how, using some different namespaces and minor tweaks, you can migrate an app from Silverlight to WinRT.
This is using ScottGu's sample search app from his blog.
Enjoy.