I got a text message from Three (my mobile carrier) telling me that Twitter is free on my phone until March 31st. It uses one of those short-code phone numbers and there’s no indication of the cost once the promotion ends.
However, given that I have unlimited (erm… 4000 I think, so not actually unlimited) texts per month, this is utterly pointless.
I may have mentioned ping.fm before. If not, then head over there, register and link it to your phone number. It’s completely free and the number you send a text message to is a genuine UK mobile number, and therefore free to send to with any inclusive package.
It does a lot more than just post to Twitter, and it’s all customisable. I’ve set mine to forward text messages to Twitter and photo attachments sent via email to ping.fm are passed on to flickr. All of this is free, and doesn’t expire on March 31st.