This is from an article just published in the Journal of Theoretical Biology (it's just the abstract; I can't post a link to the full article and quoting the whole thing isn't worth it):
"Frequency-dependent selection is an important process in the maintenance of genetic variation in fitness. In humans, it has been proposed that the polymorphism of handedness is maintained by
negative frequency-dependent selection, through a strategic advantage of left-handers in fighting interactions. Using simple mathematical models, we explore

1) whether it is possible to predict the range of left-handedness frequencies observed in human populations by the
frequency and the violence of fighting interactions; (2) the consequences of the sex differences in the probability of transmission of hand preference to offspring. We show that a wide range of values of the frequency of left-handers can be obtained with realistic changes of the parameters values. Our models reinforce the idea that
negative frequency-dependence may have played a role in maintaining left-handedness in human populations, and provide further support for
the importance of fighting interactions in the evolution of hand preference. Moreover, they suggest an explanation for the occurrence of left-handedness among women in this context, namely an indirect selective advantage through their male offspring."
Negative-frequency dependency in this case would indicate that being a lefty is only advantageous because it's relatively uncommon. Because it occurs at a low frequency, opponents in fights or sports are generally not expecting it and... ka-pow! So, there you have it. Don't believe it? Go out and sucker-punch a righty and you'll see. They probably won't be expecting it. As Bill Nye would say,"That's the sucker-punch of Science!" ...well...maybe he wouldn't say that. *I* would, though.
[edit: evidentally, we're not a minority on this web site. How does being left-handed tie in with video games and bargain-hunting? Hmmm....in that article, they go on to say that lefties only approach righties in frequency in venues involving direct, head-to-head competition, since that is when the advantage of surprise comes into play, according to their idea.]