How do you tweet someone with the mobile twitter?

[Abob]

You need to sign up with your cell phone here http://twitter.com/devices

You will get a confirmation text and from there you can Tweet by just sending a text to 40404 (in the USA). There is no charge from Twitter's end, but your own text charges will apply.

You can also turn it on and off whenever you want. Plus you can set times when you don't want to receive text messages from Twitter which is useful as you get a notification whenever there is a new direct message. Since some users have BOTS to send these messages, they all seem to come in the early morning hours.
To receive Tweets from others, go to each profile you want to receive from and turn on mobile updates.

Still confused? Try reading here... http://help.twitter.com/forums/59008/entries