Looks like a hardware problem is being encountered. Try running the Apple Hardware Test and see if you have any errors. Even if you don't, you may want to make an appointment with your local Apple Store or an AASP.
I've similar problems on my MacBook Pro13" (since OS X 10.8.1 or 10.8.2... ?).
Some kernel panics, some freezes, some sudden shutdowns.
According to the Console, running tasks at the crash moment vary:
mds, iTunes, window manager...
I ran Apple's hardware test twice, and nothing wrong has been found.
It looks like a problem with Mountain Lion drivers or Kernel