This Extreme ASP.NET article from MSDN Magazine discusses the Code-behind file and compilation changes in ASP.NET 2.0…
Conclusion from the author:
The new codebehind model in ASP.NET 2.0 seems both familiar and foreign to ASP.NET developers. It’s familiar because it still uses inheritance to relate codebehind classes with their .aspx generated class definitions, and yet foreign elements like partial classes and the implicit generation of control member variable declarations are fundamental shifts. In practice, you will probably not notice much difference in usage, but it will be important to understand the class relationships and assembly mappings outlined here whenever you are doing something out of the ordinary, like creating a common base Page class or mixing codebehind and inline code models.