As per Warren, Obama was elected by people who are by and large at least moderately pro-choice and he had the benefit of high levels of support from the gay and lesbian community. For the latter folks, legalized marriage is "the change we need." On the other hand Obama's invitation to Warren is meant to convey a kind of just the kind of post-partisanship that he promised.
I always find the Sunday political shows interesting when, in the name of decorum and public relations, people with radically different agendas share a kind of chummy, cozy friendship of old college friends. The implications are "Yes, I know you support a war in which hundreds of thousands of people will die for reasons that remain unclear, billions will be recklessly spent, and American prestige will crater but that doesn't mean you're a bad guy."
In the name of national unity, the liberal Adlai Stevenson chose the segregationist John Sparkman as VP on the 1952 Democratic ticket. Black Democrats were rightfully furious. Unity is just as often a buzzword for those quiet periods where little changes.
Rick Warren's invitation is not as bad the Sparkman choice -- he's making a prayer, not policy. But it raises the memory of groups being left in the cold in the name of unity.
Barack can have change or he can bridge the divsions in American politics. It's possible but highly unlikely he -- or we -- can have it both ways.