MovieMute is an app that silences/mutes your phone for a preset period so you can just launch and forget about it. After the time period is up, it restores the original volumes/settings. That way you don't find a whole queue of calls the next day once you've realized the ringer had been disabled.
The UI is quite simple. Just tap Turn MovieMute On to turn it on, and tada! You can configure everything in the settings, including the time the movie lasts, and whether or not the app modifies the brightness.