If I am within 25 feet of someone I find "attractive" I can simply speak.
mike007, you are a brave man :)
I think the majority of people would like to know beforehand if someone they find attractive is willing/ready to "communicate". Also, presumably bluetooth matching may suggest an ice-breaker, something better than "what's your sign".
Critical mass is a known problem, there is no magic solution. Even Nokia (with all its marketing power) acknowledges that it's a main problem for Nokia Sensor. The idea is that the cost of having it "always-on" is low so you can have it on all the time -- may not find anyone but won't hurt either.