Suprisingly, the answer is that there are significantly more hard-throwing righties than lefties. Not just in total, but percentage-wise as well.

Baseball Info Solutions has been tracking the speed of pitchers for the last few years. Overall, about half of all pitchers (54% in the last two years) average 90 or more miles per hour on their fastball. It's nice to know that the rule-of-thumb we've been using is a good one. If a pitcher throws 90, he throws hard (in the top 50% of pitchers in baseball).

But amazingly, the hard-throwing lefty is not common. Over 70% of lefties are of the soft-toss variety. Only 37% of righties are soft-tossers.

Here are the details:

Number of pitchers who throw 90 or more MPH when throwing a fastball
	Average Fastball 90+ MPH	Average Fastball Less Than 90 MPH
Right-Handed Pitchers	270 (63%)	160 (37%)
Left-Handed Pitchers	47 (29%)	114 (71%)
Total	317 (54%)	274 (46%)

Note: Includes pitchers with 100+ fastballs thrown in 2005/2006.