Doug Mataconis has the scoop.  I confess, I am opposed to the death penalty for other reasons, but I think that even if you think it's morally acceptable to execute the guilty, the long string of exonerations suggest an error rate in the justice system incompatible with executions.  Remember, most cases don't even have DNA evidence, so we're certainly not discovering all the innocent people we've sent to jail.

Forensics is an inexact science, and eyewitnesses are unreliable.  We shouldn't hand out penalties that can't be at least partially reversed if we discover that we made a mistake.