Step 1 is to get Windows running via bootcamp, parallels, or VMWare. 8)
The Mono Porject, which I've used a lot on my MBA for doing standard .Net development, has a silverlight clone project called Moonlight. Just how well implemented it is, and how cross-platform it is, I don't know. I do know there are still large portions of .Net that are not supported by Mono, that they have no plans to develop (Win Presentation Foundation, Win Comm. Foundation, among others).