OR Books has collected Tweets from the revolution in Egypt and quickly packaged them into a book, "Tweets From Tahir." The book comes out in late April, only two months after the events it chronicles. It will be issued both as a paperback and an e-book.
They are contacting the people who posted their thoughts in order to get permission, but not coordinating with Twitter. It raises the issue of who owns Tweets. Should they be treated as essays and subject to copyright, or does making them public this way, combined with how short they are, make it arguable that they're in the public domain? If it's a collection of one person's work then it seems reasonable to consider it subject to normal copyright, but what if it's a collection of hundreds of Tweets on a particular subject and only one or two from each individual? It's hard to cite "fair use" of an excerpt when the whole thing is only 140 characters long.
Interesting questions that probably will come up sooner rather than later!