They are compatible only if you shutdown the guest OS, they cannot be shared with snapshots or in the "saved" state.
Shutdown the VM down in Windows 2008, export it, then import it in Windows 2008 R2 (Hyper-V V2) and it will work. If you forget to shut it down (like i did on my first test of this) you can optionally delete the saved state and start it up from there and it will work.
There might be a patch required to allow it to go from R2 back to regular Windows 2008 but why would you want to do that?...get on R2.
http://blogs.msdn.com/virtual_pc_guy/archive/2009/10/16/hotfix-needed-to-move-r2-differencing-disk-to-windows-server-2008.aspx