Thats good news that they are going to fix it for RTM
-- Custom Page Base Classes that code-behind classes derive from that declare controls as field declarations. Because of the changes to the code-behind model, users who have migrated apps to V2 sometimes see null reference exceptions when accessing these fields on the “grandparent” base-class because the .aspx compiler can’t correctly wire-up the control references to the fields. We’ve now added support for this in V2, and the migration wizard will automatically configure code-behind classes that use custom base classes appropriately for you
I'm intreasted to know if they have fixed it in the migration wizard, which would suggest I should be possible to get it working in Beta2 or if the have changed the runtime. It would be nice if the aspx compiler could use reflection to look for matching control definitions in the base class, and if it finds them use them instead of generating it's own instance.