Yeah, 'intergration' with 2003 is really just limited to Presence info and Conferecing. You can still right-click dial users in Outlook for Communicator calls, as well as external calling if using Enterprise Voice or Remote Call Control.
Also, you can install the Outlook integration hotfix in order to remove the error message in the client, although it doesn't really do anything other than that. The Deployment documentation covers some supported features when using Outlook 2003 SP2.