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.
DNA Test Suggests Texas May Have Executed an Innocent Man
Even if you think it's morally acceptably to execute the guilty, the error rate is too high to justify the current system