I just got this:
http://www.microsoft.com/uk/mouseand...s.aspx?pid=012
I'm getting the hang of using it already, and I love how it's negated wrist movement. I have my Intellimouse Optical plugged in still, for when I game, but for anything else I'm lapping up the trackball.
Of course, I'm reserving final verdict until I've been using it a while, but thus far I'm impressed.