Michael's idea for social network software the allows you to privately message strangers in your proximity sounds like a useful application of future technology. Imagine a super bluetooth that could beam messages to people hundreds of feet away, in moving vehicles for later retrieval. When you think about it, it sounds like we're only a few years from this being common, if that long.
Then I thought about it some more, and it seems like we'd have to address all the problems we haven't solved in years of online communites. The potential problems in this offline networking would be similar to public comments on movable type blogs or postings on public telephone poles.
There'd definitely be a spam problem if every pyramid scheming neighbor could drop a note in your virtual inbox as you drove past. Heck, legit businesses would love this, imagine every time you passed a McDonalds and they tried to beam you a 50 cent off coupon. You might even have people paid to drive around and flood everyone's inbox with advertising. Then you'd get white listing like knowspam.net, with perhaps a challenge/response system to let normal folks leave you a virtual note. Driving your car might become the new friendster as everyone around you starts getting inundated with pickup lines from creepy drivers.
It seems like every cool new technology idea has to overcome what drags down older forms. Or maybe we just move on to new ideas hoping the old problems won't catch up with us.