The problem with the death penalty not being a deterrent is that's it not used often.
Texas still has a well above average murder rate, despite being by far the most prolific enforcer of the death penalty.
Even they aren't prolific enough to make it a
true deterrent. Honestly, Joe, no one will use it enough to make it one. I see it going away in nearly every state eventually.
Edit: Joe - according to this site
http://www.txexecutions.org/reports.asp?year=2011 They had 13 in 2011.....hardly enough to be a deterrent. But that's 13 more than most states so it's news. I wouldn't call it prolific though.