I've run into this same problem multiple times on a serer housing our phone system. The elapsed time continues, the byte count stops incrementing, and no tape drive activity. I cancelled the job and nothing happened and was forced to stop the BE services, shut off my tape loader, turn on the tape loader, eject the tape, and restart the BE services.
My issue still exists and I managed to find out that this file/folder is what was causing the job to hang, \\servername\c:\WINNT\system32\Wbem\regevent.mof. I've also noted a ton of DCOM errors 10010 in the event viewer on this server during the BE backup.
I've also seen problems trying to backup the system state on this same service that was caused by WMI errors. However, after numerous attempts to "fix" WMI on the server I cannot find anything wrong with it. I'd check to make sure this isn't your problem or at least get the job to stop so you can try to diagnose the issue further.