Ok, so...
Enterprise Vault 2007 SP2, Exchange 2003 SP2.
There is a clustered mailbox server which also functions as a backend server.
We've configured the back-end extensions on both nodes.
We've run the OWAuser.wsf script and the configuration script for the Backend (OWA + RPC) extensions.
We've ensured that the ExchangeServers.txt file is there on the EV server and we've created the EVServers.txt file on the backend servers (each clustered node).
We've rebooted the Admin service and resynchronized all mailboxes.
There are no extensions.
I've reviewed the hotfixes that are out there, but these do not seem to apply to the Exchange 2003 servers in question.
We've reviewed the setup log files and the troubleshooting sections of the Install & Configure guide, and there's nothing that seems to apply to our situation.
Any ideas?