BrightKite is a new social web app similar to Twitter, but instead of saying what you are doing you can add where you are doing it. (You can be specific or vague depending on privacy.) With this service, you could be walking around with your iPhone and it will give you an alert when friends enter the area. You can upload a picture of what you are doing as well.
BrightKite is Beta and invite-only. I have six invites available. Email me if you’re interested.