The Hoverstop mouse is designed to reduce the time you spend clutching your mouse by adding little breaks. If users hold the mouse without using it for more than 10 seconds, it'll vibrate and alert them to remove their hands. It looks like a good idea for combating RSI, since so many of our problems are related to holding our arms out (unnatural) for long periods, which causes the shoulders, neck, upper back, etc. to tense up. In the long run, that can be a root cause of problems like sore wrists and fingers, and even chronic headaches. I don't know if the mouse alone can serve as a good enough reminder to take breaks and stretch out your muscles, but used in conjunction with a break reminder program, it sounds like a good idea.
If the Hoverstop mouse sounds like your cup of tea, it's available over at the Human Solution. Here's a blurb from the retailer: "The system helps you to relax up to 25 times an hour. On average you will have 10 minutes of 'free' breaks each hour. That is a lot of relaxation time. Most people find it not only helps them relax, but also improves concentration on the task at hand, they get less tired and feel more comfortable."