I've got a hacked XBOX running XBMC (an open-source media center software) which I use to watch all my movies and television shows. I never watch broadcast TV.
Once I get all my stuff set up I'm planning on querying the state of the XBOX via AppleScript and if it starts playing a video I'll dim the lights in the living room. If the video is paused, I'll brighten them some until the video starts playing again. When the video is over I'll turn the lights back on and maybe have a corny applause sound or something (since my mini is in my living room).
If I want to get really fancy I can have the video pause automatically when motion is detected at the front door.
I think the automatic light dimming would be really cool though. I know XBMC sends broadcast packets about status changes but I have no idea how to grab them...I figure I'll go the reverse way and just query the XBMC status every couple of seconds.