To contribute more to this topic, though, here is a related article showing that the lefties are indeed a sinister bunch: "Gene for left-handedness is found."
I can actually throw underarm better with my left than I can with my right, and I'm right handed. I spent so much time working on my left hand throwing when I was learning to juggle large numbers, that it actually became the better hand
I broke two bones in my arm as a kid (Was originally left handed), so learned to use my right arm more.
Then I fractured my wrist, so re-learned to use my left hand, but because the injury wasn't that severe, and recuperation time wasn't that long, I was able to learn to use both equally.
Pain is the best teacher for learning to be ambidextrous.