Not 100% positive, yet... Trent might be able to give more info. We won't remove functionality... any current StrataFrame application will be able to run in either 2.0 or 3.5 (or 3.0, too). We won't be able to do the same thing that .NET did with 3.5 where they add all of the new stuff in new DLLs (like System.Core), but you won't be forced to migrate all of your existing codebase when you move to the next version of SF. So, an application might have both SF 1.6 business objects and SF 2.0 business objects.
However, SF 2.0 will require .NET 3.5 because it will require the new references for LINQ and extended generics support.