I wrote some time ago of how I would like a car's MP3 player/computer to have 802.11, so that when it parks in my driveway, it notices it is home and syncs up new data and music.
That would be great, of course, but it seems there should be other things you would do with it. Networking with the car next to you on the road seems like a cool idea but I'm having trouble dreaming up applications. Listening to the music in the next car seems cute but probably would be boring after a while. Being able to talk to the driver of the next car seems like a nice social game (and it hardly needs 802.11) and might just result in road-rage.
If common, I could see it for dating, since people seem to attach a strong romantic image to making eye contact with an attractive person in another car. There was even a dating service I read about long ago which gave you bumper stickers so you could contact somebody if you felt sparks. The personals have a section for this.
You might be able to create longer mesh networks, to share traffic info or the sort of things you used to share on CB if there are enough cars, but this would be highly unreliable, and any application here might be better served by broadcast data that goes over longer ranges. (We are already seeing broadcast traffic data services, though they will never warn about speed traps, I suspect.)
And of course, if you can connect back to the internet that's highly useful, but again this would be highly intermittent connectivity. 802.11 isn't really set up for short-burst connectivity though one could create a protocol that was, good enough to fetch live audio etc. But this ends up being just another microcell network -- what can we get car to car?
So -- all sorts of cute little applications but nothing really compelling in my view. But since we will get wireless networking in our cars for the carport sync, I invite readers to dream up some apps.