These two guys put together an iPhone app that shows you how many women have recently checked in close to you on foursquare along with their profile pictures. Oh hai stalker-y creepiness! At least there doesn't appear to be a direct link to people's profiles, but it doesn't seem like a big jump to sieve twitter and/or foursquare to find out who's there.
Things like this -- I don't even. I swear if I were a woman I'd carry a can of mace and a shirt that says "HIT ON ME AND I WILL FUCKING MACE YOU".