MrDux asks:

"..isn't that standing alone a good enough argument against the death penalty?"

No..it is a good argument for VERY careful use of the death penalty and much better investigation of crimes.

I will have to back down a bit and say that there is one other potential problem with widespread use of the death penalty...specifically, that the implementation may get into the hands of vicious opportunists who will use it for personal & political purposes.

I do NOT know how to ensure that it is used sanely and responsibly, but that does not mean that it is thereby morally 'wrong' in general.

It is an interesting bit of rationalization that it is ok to shoot a 'perpetrator' if you (or, usually, the police) can CATCH him in the act of committing certain crimes, but if he survives, or you merely capture him, it somehow becomes morally reprehensible to execute him later!
It's a sticky problem, but it deserves impartial consideration on all levels, and not just emotional reactions to biased appeals.