First... how to cause the problem.

1. Create a new C# Windows Game 2.0 project in VS2005.

If you take a look at this point, in the Configuration Manager, you will see 2 entries. One is for the Content project and the other is for whatever you named your new project. So far, so good.

2. Add a new VB.NET Class Library project to your solution. (Make sure to add it to the solution, not create a new one.)

Take a look in the Configuration Manager again. Now you'll see 3 entries, including a new one for the VB.NET Class Library you just added. You should also notice that the Content project's Platform has changed from x86 to Xbox 360. That's a problem.

Fortunately, fixing it is easy enough.

3. Click on the Active Solution Platform dropdown box and select x86 (it probably says Mixed Platforms when you look at it.)

4. Save, Recompile, etc...  you're all set.

FWIW, this has already been reported to Connect (you're welcome) and has been marked as fixed in the final version. After all, we are using Beta stuff, so it's not entirely unexpected.