I've setup a new 3.0 WSUS server, and have been successful at adding the high majority of servers and workstations.
But I do have a small number of systems that will not report in. Further searching has shown me in the registry that the WUServer (WUA) setting in the registry and the UseWUServer (0) were incorrect.
I can edit and change them to the correct values as the servers that are reporting, but when I try and force a checkin, they revert back to the original settings! I only have one GPO for all machines (I need to remedy that...inherited setup), so I'm not sure where it could be getting it from.