Hi, I thought I should tell you that I actually stopped using your site after this feature was added, because previously I could tap the bottom of the screen to get the bottom bar up, but now I can't. You might lose other users because of this, I guess.
I came up with a similar idea independently - except it was more of a social game (with foursquare-esque achievements). I envisioned a mechanism for confirmation of the rendezvous whereby both parties run the app and agree the meeting occurred and their proximity is verified. That's about as far as I got.
Edit: looks like with.me did the closest thing to this already.