SD,
Although the errors aren't exactly the same, I had issues with adding a '12 client where a '07 was previously installed. I too wasn't able to assign a site code. Check the client's registry to make sure it's not getting an old site code. HKEY LOCAL MACHINE/Software/Microsoft/SMS/Mobile Client, Key Name: Assigned Site Code. If it's pointing to the old site, delete it and try reassigned the client through the client's CCM GUI.