I ran into this little devil during my first installation of Windows 2008 Small Business Server (SBS). The setup failed with the error message:
FATAL: DcPromo_JoinDomain: The server was not promoted to a domain controller.
The solution was surprisingly simple. An active network connection is required to start the setup, however I had disconnected the NIC during setup. Bad move, this causes the setup to bail out. The solution, of course, is to make sure the network card remians connected for the duration of the setup.