I'm right handed but use a left-handed mouse. I started doing this after I got tendinitis for the second time. All servers and most of the computers at work were set up for righties, so by using a left-handed mouse on my own workstations, I was able to balance the load and thus take some of the stress off my right wrist.
It's nice using a left-handed mouse with a 104-key keyboard, as it gets the num pad out of the way and lets me have my mouse closer to the alphanumeric keys. Plus, I can mouse with my left hand and write with my right! Much more ergonomic it seems.