At the last PDC (can't remember which talk it was) they gave us the information that it will be possible to share assemblies between regular .Net 4 and Silverlight 4.
Unfortunately I can't find anything on this. Was this feature dropped? What options/limitations are there?
(There are similar questions on SO but they don't say if they apply to SL3 or 4.)
See Sharing Silverlight Assemblies with .NET Apps from the CLR Team Blog.
Basically if your assembly only references:-
That assembly can be shared by both frameworks with each framework using its own version of those references (that is .NET won't be attempting to load Silverlight's version of System.Core for instance).
Of course you will still need to limit your code to features of these references that are actually common across to the two frameworks.