Similar Experience - Heat, too
Similar experience with inconsistent circumstances. Have had two lock ups on shut down, lock up and infinite spinning beach ball on attempted restart, had to force hard shut down each time to recover. Tried to restart and fans came on at full throttle, then I noticed that the underbelly of my MBP was very hot. After things cooled down overnight, was able to restart.
Had a lock up yesterday morning with several Safari windows open, a few other apps and finder window; seemed pretty benign; did three-finger swipe for Mission Control and the system locked up.
Scared me into going back to my backup clone that has the latest SL. Searched around for threads with similar problem all afternoon. There are others having similar lock ups. Pre-Lion install recommendation was to use Disk Utility to Repair Disk Permissions. Gave that a shot and decided to try installing Lion again.
For good measure, performed Repair Disk Permissions after fresh install. I'll caveat that I'm not convinced that was the problem or fix, but Lion working fine for now, and I was able to shutdown last night without problems.
Have RARELY had the system lock up with SL any other circumstances. Now I'm a little terrified each time I power up or down. I want Lion to work, but am not thrilled so far.
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15" 2010 MBP 2.4 GHz Intel Core i5 8Gb RAM dual SSD; iPhone 4; iPad 2