While twitter asks "what are you doing?",
brightkite asks "what's happening?"
A small distinction... but one that might make a big difference in the world of mobile social networking, because rather than follow your friends' activities, you're finding people and activities that are close to you. Right now.
I know that Fej has been using the SMS-based version of brightkite for a while (he told me about it months ago), but I didn't jump in until the iPhone app was launched a couple of days ago. It uses the iPhone's GPS in a very impressive way.
Obviously, it's still early and so few of my friends are using it so it's still just a novelty... But I do have 3 invites available, so if anyone is interested, let me know.
At this point, it's so easy to use that I think I'll be using both twitter and brightkite, at least for a while.