Michael Schwarz, the creator of Ajax.NET, the first .NET library to enable Ajax, just wrote on his blog a nice feature to feature comparison of MS Silverlight and Flash/Flex:

Rich Media Platform Comparison

I don't get the why he (or better the original source) included in the comparison also .NET 3.0, which is a completely different beast.

Anyway it turns out that they are basically the same for Rich Applications, but Silverlight, being based on WMF, has a lot more features (DRM, HiDef, production tools) than Flash in the video field.