Back Talk has a couple of articles (here and here) on the death penalty, specifically on whether it deters murder. It's always made sense to me that the threat of losing one's life must be a deterrent to a behavior, despite claims to the contrary. As these articles report, the science bears this out, concluding that 3 to 18 lives are saved for every person that is executed.
According to the articles, about 60 people are executed in the U.S. each year. Of these, fewer than one turns out to be innocent. This means that the death penalty saves between 180 and 1080 lives for each person wrongly executed. While the execution of an innocent (of the murder in question, at least) person is tragic, it must be balanced against the hundreds of lives saved.
So while there may be other sound arguments against the death penalty, the desire to save innocent lives isn't one of them.