I have Exchange 2003 and Exchange 2010 running in co-existance.
Everything works inlcuding bi-directional email flow between the two and OWA.
The difficulty now is that email flow from Exchange 2003 and 2010 is slow. Users are reporting delays of 10-20 minutes. Emails from Exchange 2010 to 2003 is immediate.
Checked the Exchange 2003 Routing Group Connector queue and it routinely has messages whereas the other queues are empty.