I've just had my first Mountain Lion lockup. The screen was entirely frozen in place (I have seconds displayed on my clock in iStat Menu and even that wasn't moving) BUT the mouse still worked. I could move that around. Nothing else did anything.
I noticed that the keyboard backlight also didn't come on if I typed anything in.
I have SSH access to the machine and that also still worked so it looks like just the GUI was the problem.
Anyone else had this, and have a fix if it happens again?
Its a MBP Retina, 2.6/16/512, with the SMC update from yesterday applied. No optical bay. Graphics: Intel HD4000 integrated, and a discrete Nvidia GT650M.
At the time of the hang it was on mains power, undocked, no external devices plugged in. The NVidia GT650M was in use (it always is, some of my running apps force it on for some reason, postbox for example).
I had Firefox Aurora (their bleeding edge version) active and it was actually in the middle of rendering an Ebay login page as well.