Let\'s imagine I already have a project building .NET 3.5 assembly. Now I want to build this assembly for Silverlight, and moreover, maintain its Silverlight version with minima
I concur with Scott. Save yourself a lot of pain. Two projects that share the same codebase is the way to go. You'll need it to use VStudio in both environments, to use different libs, to include/exculde files, to do so many things...easily!
The reason's for having two projects far outweight the excuses for having one.