There are gobs of mice on the market and many of them claim to be ergonomic and help to prevent repetitive stress injuries. The catch with some of the portable mice that are around is that the things are only for right-handed users. If you are in the market for a new mouse for your notebook, you might want to check out the Smartfish Whirl.
The mouse has a very unique design that looks like a normal mouse set on top of a stand and it is ambidextrous. The idea is that the mouse section you put your hand on can pivot in any direction for more comfortable use and to allow a more natural hand position. That should mean less stress on your wrist.
The mouse has an 800dpi laser engine and uses 2.4GHz wireless connectivity. It will work with Mac and PCs and you can order one up in several colors now for $49.95 (USD). Colors include Carbon Fiber, Black, Blue, Silver, Red, and White.