I searched the forums and it looks like this bug was supposed be to fixed in 3.0.1, but although I have that version (3.0.471.2 according to Resharpers About page), I am seeing the error.
Have PageBase.cs in App_Code directory.
Inheriting from this class in web root directory is fine.
Move page into a folder, and the class cannot be found. It compiles fine.
What can I do?