Article in Guardian online on 15 August 2006.
Warns that sending lots of text messages can cause pain and swelling of tendons at the base of the thumb and wrist. It seems mobile phones are not designed with the capabilities of the thumb in mind, which is "not a very dextrous digit."
However, a reader has added comment. "Please tell me this is a joke article. I really hope there hasn't actually been serious research done" on this topic.