I switched my mouse over to the left side of my keyboard (despite being decidedly right-handed) to avoid the numberpad issue. In the end I still had to ditch straight keyboards to avoid RSI.
(I've since gotten a keyboard without a numberpad and switched back, but I can still use a left-handed mouse pretty well. It's one step below having a trackball as far as confusing people who try to use my computer goes. :P)
I usually use a thumb trackball, but once when my wrist pain got pretty bad I started using one of the ambidextrous trackballs with my left hand for a bit, it was actually pretty nice.
(I've since gotten a keyboard without a numberpad and switched back, but I can still use a left-handed mouse pretty well. It's one step below having a trackball as far as confusing people who try to use my computer goes. :P)