OK. I get that you can run a generic Silverlight 4 app on any platform you want. I've tried it and it works.
In my case, however, I want to take advantage of the new features in VS 2010 and Silverlight 4, including WCF RIA Services. To do this, I'm pretty confident I need to use the .Net Framework 4.0. But when I set IIS to use the .Net Framework 4.0, my ArcGIS 9.x map services fail. If I try to build the Silverlight 4 site using .Net 3.5, I have to utilize the older WCF RIA Services (Beta) that worked with .Net 3.5 but will only be usable for a few more months (see quote below).
Curisously, my development machine works perfectly using VS Development Server inside VS2010.
This is from Scott Guthrie's Blog:
".NET 4, VS 2010 and Silverlight 4 have key features that we could not have done without (e.g. partial trust capability in .NET 4 and non-exception validation in Silverlight). Given that, we decided to focus on providing the best experience on the current platform and not evolve .NET 3.5/ Silverlight 3-based version further. It was a difficult choice that we arrived at after much discussion on the RIA Services forum and advice from a number of early adopters. The version shipped at PDC 2009 is still usable for a few more months if you need time to migrate."