Hmmm.. using the leds on the rear of your car to message the person behind you. Of course, with the limited space, you'd probably be restricted to 8 characters. That could be interesting. I can see the headlines now:

"Man brutally beaten after child in passenger seat types 'EATME' into his rear-messaging system"

"Man maced after typing 'UBLOME2' at hot blonde in car behind him".

I'm not going to even try to go into all the 8 character phrases this sytem could be abused with. Now, a system that transmitted phrases to another car within a short distance, via radio, would be useful. Perhaps it could be based on the license plate of the car you wish to send to. When you install your unit, you program it with your current license plate, and when someone within say 100 feet wants to send your car a message, they could just address it to your license plate as an ID, and it would transmit the message encoded by your plate number. The recieving car would listen to all traffic, see its own license plate ID go by, grab the message, and display it on a scrolling screen, perhaps built into the dash. This would be useful for stuff like sending a message to a car infront of you saying "your left rear brakelight is out" or whatever.. since people rarely notice things like that for a long time. Then again, there would always be those people who abuse the system, and rewire their listening unit to recieve all traffic going by, intercepting communications meant for other cars. (kind of like setting your network card into promiscuous mode). They could of course design it using PGP to avoid this... But that wouldn't address the issue of radio-spam. Think about it - if a system like this ever caught on, there would be people who built a custom addon to their unit that grepped car id's around them and then sent spam to them. Probably would forge the headers to use another car's ID closeby also. "Do you need an oil change? Just swing on by Bob's Shack and he'll hook u up real good for $9.95." Aaagh. That would just be wrong. =( hehehe. Anyway, I'm getting off on a tangent, so I will stop here. =)