O2 links with twitter to make tweets go mobile and free

To set Twitter texts up, head to Twitter, choose Settings then Devices. Once you've followed the prompts there, you can then choose which followers you want to get texts from. You'll then receive each of their status updates by text message - you can also choose to receive every direct message you get too.
From today you can also send Twitter updates to 86444 - texts to this number will cost your normal text message rate or be part of your inclusive bundle.

Q: How much does it cost to receive Twitter updates by text?
A: Nothing, it's free to receive Twitter updates by text message.
Q. What kind of Twitter updates can I receive?
A: You can receive status updates and @mentions from friends you have turned on device updates for, and direct messages.
Q. Is there a limit?
A: Yes, we have a fair use policy. People who receive more than 600 messages per month will be asked to moderate their usage. Otherwise, it's going to impact the experience for everyone else.
Q. Can you send tweets and reply to messages by text?
A: Yes, you can send updates, direct messages and control your device notifications by text message. Check out http://help.twitter.com/forums/10711/entries/14020 for a list of command codes. All texts are sent to 86444 and will be charged at the normal text message rate or come from your inclusive text bundle.
Q. Is there a limit on sending messages?
A: No, you can send as many as you're happy to pay for.

source: http://dnc.o2.co.uk/home/2009/07/twitter-goes-live.html