I would just like to point out why it can't be used in VS 2005/2008 .

The content-pipeline which is used for loading resources such as models is only accessible in C# express. The framework of XNA is accessible from VS 2005 though.

I am saying this to point out that if you do all content-pipeline related stuff in C# Express and wrap it in a Code Library (DLL) you can probably do all the rest of your coding in VS 2005/2008 even with the current XNA version, just by adding a reference to the library. The disadvantage is of course that you can't add any new resources from there (such as more models).

Also just in case you came up with the "brilliant idea" of adding those resources at runtime, without use of the content pipeline: you can't, impossible. The XNA framework uses a lot of sealed classes (so that you can't override them) and it also has a lot of internal constructors so that some object (such as ModelBones) can only be instantiated from inside the XNA framework. Either way, it would be too slow.