A few people on the Apple Discussion boards have described an issue that seems to have cropped up in OS X 10.6.5, where the system may force-quit the log-in session when attempting to play video.
This is not a widespread problem, but has affected a few people and seems to happen when they use certain media players that support hardware-accelerated video playback. When the crash occurs, the system may flash a blue or black screen and then present the log-in window, quitting all user applications and unsaved data. Not all video players spur this behavior, and the problem appears to happen randomly, but so far users have found that builds of Flash and VLC media players have caused this issue.
This is likely a driver-related incompatibility with OS X 10.6.5, especially since some users who have tried updating to prerelease versions of OS X 10.6.6 have found running the same applications no longer results in a crash.
Generally, if a fault is with an application, then the application itself will shut down, leaving the log-in session and other applications running. However, the log-in session suddenly quitting indicates that a core system process that either the user's "launchd," "loginwindow," or "WindowServer" processes depend upon has failed. A likely reason for this is driver-related malfunctions.
For now, if you are experiencing this problem, try using a different version of your media player, or optionally turn off the hardware acceleration features of the player. It looks promising that the next OS X update will address this issue, so stay tuned for its upcoming release. Apple has been releasing more builds of the upcoming 10.6.6 update to developers, and there are news reports that Apple is trying to get the Mac App Store feature (rumored to be supported in 10.6.6) up and running by December 13, which suggests the update should be out in the next week or two.