As a trained ergonomist with family members with severe repetitive stress injuries like carpal tunnel syndrome, I've long wondered when the first mobile-related RSIs would show up. I knew it would be a while because few people use their phone for hours without break, but some people do.
I was particularly worried about holding a phone up to an ear for hours (one reason why I don't like finger phones) because of the major strain on the elbow and the fact that people do talk for hours. This, to my mind, is the primary value of hands-free headsets.
Well, among youth there are certainly people who type (or game) extensively. There's been evidence of use-driven structural changes in the hands of gaming and texting youth. This has been recognized by the term "Thumb Generation" ("Thumb Tribe" in Japan). However, nobody had yet talked about RSIs among this group.
I've now seen my first mobile-related RSI story, from the CBS Chicago affiliate. Indeed, people are getting RSIs in the elbow due to talking on the phone. There are even cases of injuries induced by typing on the Blackberry.
So everybody: use headsets or a speakerphone if you talk for long periods. If you don't, switch ears periodically. Take longer breaks between using the phone ... and don't use the computer mouse or mobile phone keypad to fill in the time. And take a break from the typing.