While Silverlight is a functional subset of WPF and the gap is ever narrowing, there are many features that WPF supports that Silverlight does not. Silverlight even offers a few features that WPF doesn't. Microsoft asked us to do a whitepaper on the differences. Sergio Loscialo, with help from Steve Porter and Kate Gregory, has produced a 69-page paper that goes into amazing detail comparing and contrasting the differences. With this paper, you can learn how to write code that's portable between Silverlight and WPF. Read Guidance on Differences between WPF and Silverlight on CodePlex.