I'm saying I think Apple will build Bump into the phone. Then it's game over for all of this.
As far as I've been able to find the only two mediums that match that friction are an email or a phone number. (And we're looking at adding text message support instead of just email)
A third medium is that most people have the ability to access a website. I built http://near.im/ (sorry for the ugly frontend) with this in mind. An advantage here is you don't have to communicate to another person your name or e-mail address (which can be hard for some people), just describe a short domain name.