Here is Obama's explanation:
http://www.youtube.com/watch?v=syIEoSIJHis
I am beginning to form a new theory about it, not necessarily one I agree with on timing, but I suspect that Obama is concerned about those Americans who cannot think for themselves, and there are millions of them. If Warren speaks in the same public forum as Obama and many others, and pulls his usual sociopathic ass-hattery, then perhaps some of those Americans will see for themselves Rick Warren's message of hate and cherry-picking of scripture. Perhaps by shining a bright light on the contrasting messages in full public view we can get some decent public discussion going about these people who preach hate. Obama might be sending the message that we all need to start pulling our own weight. We supporters need to actively combat ignorance and stupidity, and not remain passive to the aggressive onslaught of hate-mongers in America.
Full Disclosure: I actually own a copy of Warren's book "The Purpose Driven Life", given to me by a very confused brother one year after he discovered I was Buddhist. The brother is still very lost and confused and thinks I am damned. The book is horrible beyond belief, but I keep it to remind me how messed up huge portions of our country's people are and how easily they can be misled.