In the "Sources" menu it displays as connected, but a box pops up and says:
no signal, check cable connection and settings on connected device
My PC running Ubuntu 12.10 is set to mirror displays when I connect to it.
I believe this is because you probably need to upgrade some of your drivers for your graphical card. Which garphic card do you use? NVIDIA, ATI, or INTEL? (Or a mix of many?)
Another possible source of the problem is the that the cable is somehow damaged, but I believe you have that covered :P