I’ve no idea how common it is for TVs to have a sleep timer function; my Philips TV/VCR combo has one. You just press a “Sleep” button to set a sleep timer in 15 minute increments. After that time, the TV is shut down after a brief countdown.
This can be exploited for reminders – if you’re watching one channel, but want to be reminded to switch to another channel in a while (in about 15, 30, 45 minutes and so on). I’m using this right now since Seinfeld was shifted some 30 minutes ahead because of a sporting event. Since iCalTV is thrown off by this last-minute scheduling change, setting a 30 minute sleep will remind me when it actually starts, by the TV counting down or turning off.