Rather than calling somebody or sending an e-mail or a Twitter or an IM, you just open up your contact list and click on their name. Wherever they are, your communication reaches them via the most convenient and appropriate means. So, they're walking on the beach, their iPhone rings. If they're in a meeting, they get a text message. If they're at their desk, they get an e-mail. If they're in Asia, they're probably asleep, so they get a voicemail.