Async communication is key to twitter. It's also cool that you can tweet by any method anyone has ever thought of, from almost any device that can send any sort of data whatsoever. The API has helped them greatly. I'm sure that someone, somewhere has a computer that transcribes short voice messages that can be left by phone. If not they will by the time I hit "reply" ... there should be a rule #34 for twitter.

* e-mail

* sms (key for reaching mobile people w/o smartphones)

* web browsers

* native apps / widgets

And then services like twitpic built around twitter.