This update addresses an issue where MacBook Pro (15-inch Mid 2010) computers may intermittently freeze or stop displaying video.
For detailed information about this update, please visit: http://support.apple.com/kb/TS4088.
Here is a bit more from TS4088, via the Wayback Machine:
Apple has determined that a small number of MacBook Pro (15-inch, Mid 2010) computers may intermittently freeze or 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.
The NVIDIA GeForce GT 330M was at the heart of this issue, and is one reason Apple and NVIDIA still don’t work together.