Our doc team has outdone themselves again. Only a few short weeks after the annoucement of Windows Server 2008, you have some documentation in the SBS Tech Library on how to join it to your Small Business Server 2003 network.
The direct link to the topic is available here.
As a heads up, you'll be unable to use the http://servername/connectcomputer link to join it to the network, so make sure you check out the help topic to save yourself any pains.
Thanks to the SBS documentation team!