I am not any sort of expert on the subject, but I'd really suspect that designing a mouse so that your fingers do more of the work is the exactly wrong idea. You want to be using your arm for the majority of mouse movements, as it's much better at resisting RSI injuries.

That said, I really used to like my logitech trackman marble, which was all thumb movement. I abandoned it when I moved to a laptop, and now use a traditional mouse on desktops.

I also throw out non-optical mice whenever I come across them at work.

Matthew