Wow. I knew it was only a matter of time before we started sharing where we were at any second.
Brightkite lets you “check in” your current location, and see who is around you within a certain radius, you can choose from 20m to 100km.
There are privacy settings of course, and you can choose a level where your specific location is never given out.
Use it with your cellphone (yes, a normal cellphone), and of course iPhone is accomodated for too.