You've repeatedly stated the argument that capital punishment isn't a deterrent. This is the often cited "conventional wisdom." However, it may not be true if the authors of this study are correct. Here's the executive summary:

Recent evidence suggests that capital punishment may have a sugnificant deterrent effect, preventing as many as eighteen or more murders for each execution. This evidence greatly unsettles moral objections to the death penalty, because it suggests that a refusal to impose that penalty condemns numberous innocent people to death. Capital punishment thus presents a life-life tradeoff, and a serious commitment to the sanctity of human life may well compel, rather than forbid, that form of punishment. Moral objections to the death penalty frequently depend on a distinction between acts and omissions, but that distinction is misleading in this context, because government is a special kind of moral agent. The familiar problems with capital punishment - potential error, irreversibility, arbitrariness, and racial skew - do not argue in favor of abolition, because the world of homocide suffers from those same problems in even more acute form. The widespread failure to appreciate the life-life tradeoffs involved in capital punishment may depend on cognitive processes that fail to treat "statistical lives" with the seriousness that they deserve.

Before you dismiss this study as the product of some "right wing" radical, it comes from the liberal Brookings Institution. The authors are from the not exactly conservative University of Chicago.

There's also a good letter from a prosecutor to a blogger who opposes the death penaty here. He makes some interesting points.