Funnily enough, I don't have any strong opinions on this topic (much like abortion). I'm not quite sure what all the fuss is about, actually. Both sides seem to assume that capital punishment is far more serious than other kinds of punishment. Only execution gives the worst offenders what they deserve, or violates their human rights, depending on who you ask. But I'm not sure that it's so different from life imprisonment.
It's terrible to take the life of an innocent person, of course. But is taking away their freedom really all that much better? Injustice is terrible no matter what form the misdirected punishment takes. I wouldn't be surprised if some people would rather die than spend the rest of their life in prison. After all, as liberals are usually well aware, it's our quality of life that matters. There's more to life than having a heartbeat, and we tend to be critical of conservatives when they forget this. So why are we suddenly the absolutists when it comes to capital punishment?
As a utilitarian, I hate prisons - they're so damn wasteful. In the linked post I suggested some alternative punishments for minor crimes (it might even be worth considering corporal punishment - as Ethical Werewolf commented, "I'll trade an inhumane, inefficient system for an inhumane, efficient one any day."). Perhaps capital punishment would be a more efficient alternative for the most serious crimes?
So, for me at least, the question of capital punishment is less a moral issue than an empirical one: would it have the best consequences? Would it be cheaper than life imprisonment? Would it be a better deterrent? Or would it just encourage more violence and a callous attitude towards life in the broader culture? It could well be that it would do more harm than good. But this is an empirical question which requires careful research / examination. I'm not convinced that we can rule it out a priori. Or am I missing something?
Update: Matthew Yglesias makes a similar argument.