My mid-2011 MBP (Core i7, Lion), seems to fart around 4 or 5 times a day. It usually happens randomly. There doesn't seem to be any issues in Console before the "boot" line.
What happens is that computer stops responding, including mouse, lights, function keys, etc. The only thing to do is to turn it off manually and back on again.
EDIT DEC 1, 2011 for the last 3 weeks I haven't had a single freeze. It all happened after I updated my OS X.
Best Answer
This is almost certainly some kind of hardware problem. It could very well be your hard drive, as @M123 suggests. It could also be memory or even something wrong on the logic board. It's hard to narrow down without further tests, but with mouse and everything freezing, it's almost certain a low-level lock up.
What is your situation for bringing it to an Apple store or getting some diagnostic tools?