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.