I have been unable to find any direct promise by President-elect Obama to retain or abolish the death penalty, but according to Wikipedia, Obama has said that the death penalty is used too frequently and inconsistently. However, he favors it for cases in which "the community is justified in expressing the full measure of its outrage." [1]
Speaking as a state senator about the Illinois legislature's constant additions to the list of factors that render a defendant eligible for the death penalty, Obama said, "We certainly don't think that we should […] have this laundry list that does not make any distinctions between the run-of-the-mill armed robbery that results in death and systematic killings by a terrorist organization. And I think essentially what the reduction of aggravating factors does is, it says, 'Here's a narrower set of crimes that we think potentially at least could deserve the death penalty.'" [2]
In his book "The Audacity of Hope" Obama wrote, "While the evidence tells me that the death penalty does little to deter crime, I believe there are some crimes — mass murder, the rape and murder of a child — so heinous that the community is justified in expressing the full measure of its outrage by meting out the ultimate punishment. On the other hand, the way capital cases were tried in Illinois at the time was so rife with error, questionable police tactics, racial bias, and shoddy lawyering, that 13 death row inmates had been exonerated." [3]
On June 25, 2008, Obama at a Chicago news conference condemned United States Supreme Court decision Kennedy v. Louisiana, which outlawed the death penalty for a child rapist when the victim was not killed. He said that states have the right to consider capital punishment, but cited concern about the possibility of unfairness in some sentences. [4]
"I have said repeatedly that I think that the death penalty should be applied in very narrow circumstances for the most egregious of crimes," [4]
"I think that the rape of a small child, 6 or 8 years old, is a heinous crime and if a state makes a decision that under narrow, limited, well-defined circumstances the death penalty is at least potentially applicable, that that does not violate our Constitution." [4]
Sources:
1. Religion and Politics 2008: Death Penalty Profile
2. "The NewsHour with Jim Lehrer", transcrip (June 10, 2002)
3. The Audacity of Hope, by Barack Obama, p. 58 Oct 1, 2006
4. Obama disagrees with court on child rape case - June 25, 2008, Chicago