For the "Interop RGC" to work correctly for delivery of mail from Exchange 2007 Hub Transport Servers to Exchange 2003 servers, the Virtual Server used for Exchange server communication on the Exchange 2003 servers MUST be bound to the IP address of the server itself. It cannot be bound to an extra IP address.
Even though the Routing Group Connector in the 2007 Exchange Administrative group Routing group appears to specify the correct virtual server on the 2003 bridgehead, it doesn't use it. The 2007 Hub Transport server appears to ignore this, and will instead talk to the IP address of the Exchange 2003 server directly.
We almost found this out the hard way.