Sociologist Mimi Ito has just published a great paper on the way that mobile phones change teen social interaction on Vodafone's Receiver magazine:
After young people have converged in physical space, mobile communication does not necessarily end. In contrast to work meetings in which mobile communications are largely excluded, among gatherings of young people, the mobile phone is a social accessory. They might call a friend, inviting them to join them, or getting information about the conversation at hand. When an email message comes into a friend's mobile, it is quite common to ask who it was from and a conversation about that person to ensue. Young people generally reported that they had no reservations about making contact with others via mobile phones when they were with a group of friends, though they might make a brief apology if a one-on-one gathering was interrupted with a voice call.
(via Joi Ito)