A virtualization MVP recently bought this to my attention:

In Windows Server 2008 R2 there was a minor change to the format of differencing virtual hard disks.  As a result you will need to apply an update to Windows Server 2008 if you want to move differencing disks from Windows Server 2008 R2 to Windows Server 2008.

You can download this update from here:

http://support.microsoft.com/kb/971677

Note that this applies to Windows Server 2008 and Windows Server 2008 SP2 (not to Windows Server 2008 R2).  If you do not have this update installed and you try to use a differencing disk created with Windows Server 2008 R2 you will receive an error message that states that the virtual hard disk is corrupted or unreadable.

Cheers,
Ben