Similar topic, different application, so much fun - have you tried TV-B-Gone for the N900? It's a one button application. You press it while facing the TV and in less than a minute the TV in front of you switches off. It works on all TV's in Europe and the US, depending on the setup you have chosen.

How does it work you may ask? It sends +- 200 remote control signals to a TV in the hope that one of them belongs to that TV. It's worked on every TV I have tried.

Places you can have fun with this application, with little detection.

1. Television Stores
2. Pubs showing boring sports (e.g. Snooker, Football, Rugby, Cricket).
3. Cafes
4. Restaurants (because you are there to eat not watch TV).
5. Trade floors in busy investment banks
6. Waiting rooms (Dentist, Doctor, etc).
7. Airports
8. Modern Stations

Obviously be careful, some people might not see your practical joke as being funny.