I just came across a blog called Disabled Hands that has tips for reducing the occurrence of BlackBerry Thumb. The tip that I found the most interesting was using a smaller BlackBerry, such as a 8700 model.

Switch to a smaller Blackberry. One of the main reasons for Blackberry Thumb is the over extension of the thumb, especially true in the bulkier devices like the 7520. To prevent this you should consider switching to a smaller device like the 8700.

My guess is that the same would hold true for 71XX series BlackBerry devices as well. I have used and 7100 for over a year and can honestly tell you that my thumbs have never hurt.