I just finished reading this post from the BCLTeam blog on the Silverlight 1.1 Alpha release that was announced at MIX07. It sounds like there is a new security model that is being introduced with Silverlight as a replacement for CAS (Code Access Security). I haven't done any research yet, but it looks like we'll be able to just decorate methods with attributes like Transparent or SafeCritical.
From just a cursory glance, it sounds like this will be much easier to work with than CAS. I couldn't ever really follow CAS very well - usually I just followed the FxCop guidelines and left it at that. I found the MSDN documentation on developing Silverlight applications with the .NET Framework, but there isn't much on the security side of things yet. I'm interested to see if Microsoft will start to move away from CAS to a more developer-friendly model.