Apple has determined that a small number of MacBook Pro (15-inch, Mid 2010) computers may intermittently stop displaying video on the built-in display or on an external display connected to the MacBook Pro. In this situation, you may also see a restart warning message before the video is lost or the display turns black or gray. Affected computers were manufactured between April 2010 and February 2011.
Via Apple Support
Update: The MacBook Pro intermittent black screen issue has been addressed with the release of MacBook Pro Video Update 1.0 — but it's only good if you're running Lion (10.7.2+). If you choose to remain on Snow Leopard, you should make a Genius Bar appointment at your favorite Apple store to have the problem addressed. Chances are you'll get a new motherboard out of it. See the support article for more information.
It seems the MacBook Pro/Lion Black Screen of Death™
I updated my Lion partition to 10.7.2 yesterday, and it seemed more stable than it was before - though I didn't get much of a chance to really push the system. I was planning to update my production system this evening, but I think I might hold off a little longer until there's more info from Apple in the article.