I actually agree with virtually everything Scott said here, and I also must admit that many of the criticisms of Warren have been off base. However, I do have two concerns about Warren's decision to allow Obama use of his pulpit.
First, although I agree that you should not have to pass a religious litmus test to join in the fight against injustice, from what I understand Obama did use his opportunity to speak of his faith and convictions. His inclusion and discussion of his faith makes it appear that his pro-abortion choice advocacy, including having his wife sending out a fund raising letter in favor of partial birth abortion, is simply a in-house discussion amongst brothers. In other words, allowing Obama to speak of his faith in that context masks the clear differences in worldviews. If he wishes to limit the discussion on HIV, I would have no concerns.
Second, it seems the main reason Obama was there was to appeal to evangelicals and soften his clear opposition to many things that are Biblically sound. Here is a sample of a Time article describing what Obama accomplished: The invitation works perfectly for Obama. Through his autobiography The Audacity of Hope and his public statements, the Senator had already positioned himself as one of the rare potential Democratic Presidential candidates who can truly talk the Christian talk. Today's speech can only reinforce that impression. Says Collin Hansen, an associate editor at the Evangelical monthly Christianity Today, " I think the Senator's political team, or whoever's making the decision, was smart to associate him with Warren. It suggests that there are Evangelical moderates that they can work with, or reach, or maybe even attract their votes." We should not be interested in helping a politician who will advance the cause of evil if he reaches his goal of the presidency to fool us by "talking our language". If this places Obama one step closer to the White House, I believe it may not be a such a noble idea.
In short, we need to accept the help of others who differ from us theologically and in some ways ideologically if they wish to fight for justice. Nat Hentoff would be welcome to speak about abortion in my church anytime he would like to. However, we need to be careful that we are not, even unsuspectingly, providing one who clearly promotes evil an avenue to do so.
Update: Case in point. This story from the Chicago Tribune. In the approximately 25 paragraphs exactly one discusses the AIDS epidemic. The rest of the story speaks of Obama's attempt to court evangelicals.