E' alla fine di un luuuuuungo post, ma penso sia interessante:
There is one caveat to this - called "red bits". These are the changes in the framework that are part of .NET Framework 2.0 Service Pack 1 and .NET Framework 3.0 Service Pack 1, both of which will be released along with Visual Studio 2008. These two service packs are required for .NET3.5 to be installed, and they include a few very targetted additions of new functionality to the .NET 2.0 and .NET 3.0 frameworks. This means that when you target .NET 2.0 in Visual Studio Orcas, you are really targeting .NET 2.0 SP1. This is similar to what happens when you are using Visual Studio 2003 after the .NET Framework 1.1 service pack is installed on your machine - so it's really nothing new. But for those who want to be extra cautious about their framework dependencies, it's somthing to be aware of.
Fonte: .NET Framework Multitargeting in Visual Studio 2008 (aka Orcas)