Not to be argumentative but doesn't shaking present the same scaling problems? You can only stack so many iPhones before it becomes dangerous to shake them (for me shaking one is enough!). Why not put a button with a certain timeout, surely users can hit the button within 5 seconds of each other.
We're not talking about scaling by syncing more than 2 devices simultaneously. We're talking about scaling our user base.
The problem is that we can only expect GPS to be accurate to 1-2km and the phone clocks are often off by 1+ seconds. This makes it difficult in metropolitan areas when multiple transfers can occur simultaneously within a 1-2km radius.
I was joking about shaking one, but stacking two and shaking makes me uneasy because I am paranoid about scratches. And yea, throwing or more simply dropping is a slight concern.
