Twitter have updated their terms of service to make it clear that whoever posts the tweet owns it. This is timely as I just wrote about who owns forum posts. See the Twitter announcement here. Apparently the reason for clarifying the issues for Twitter has been the publication of books that are a compilation of tweets (I read and recommend: Twitter Witt). They state in the blog post that “Twitter is allowed to “use, copy, reproduce, process, adapt, modify, publish, transmit, display and distribute” your tweets because that’s what we do. However, they are your tweets and they belong to you.”
If you have no idea what Twitter is and why its an important promotional tool for forums, then see the articles tagged with Twitter.