You've probably seen Bump advertised on the iPhone ads as a way of sharing contact information, which it is certainly good for.
However it can also be used to pass along photos and other things with a single simultaneous shake or "bump" of the iPhones. I got a chance to test it with my friend's iPhone, as he deleted his own contact information, and wanted it back.
However he had to send me something too, a photo I think. There was no option of leaving one side to do an empty bump as might be required at times.
There's certainly some good ideas and it's an ambitious app but it'll take a little time and a few updates to become a neccessity for all iPhone users.