Here are a few things I've learned about the Class Designer and ASP.NET Projects:

1. The Class Designer will *not* pick up classes defined inline in a .aspx page
2. The Class Designer will pick up classes stored in the new /code directory (read more about /code)
3. If no class files are present in your ASP.NET project and you try to create a class diagram, Visual Studio will only let you create a C# class. This is a known issue and will be fixed going forward. Again, this is only an issue if you have no class files. To work around this problem you simply add a .vb file to the project and all is well.