Trying to join a Server 2003 machine to the domain via the SBS Connect Computer application resulted in the following error:
This computer does not meet the requirements necessary to connect to the network. Supported Operating System Not Found. To joing your computer to the Windows SBS network using the Connect Computer program, you must be running...
Simply add the Server 2003 computer to the domain manually using the "Computer Name" tab in "System Properties". Once the member server has been added, go into Active Directory Users and Computers and move the server from the SBSClient OU to the SBSServers OU. This TechNet document is my source: http://technet.microsoft.com/en-us/library/dd469602%28WS.10%29.aspx
The Long Story:
While attempting to join a Server 2003 machine to my new SBS 2008 domain, I received the above error. For a split second I panicked thinking that for some strange reason Server 2003 could not be joined to the domain. I knew this to be absurd... and yet I didn't want to assume that Microsoft was more sane in some of its practices than what prior experience had proven. Some quick Googling proved that indeed Server 2003 can be a member server. However, the only trick to the process is to make sure you move the newly joined server to the SBSServer OU to keep the client PC GPOs from applying to the Server.