To be honest, I don't see what the fuss is about.
I would not be offended if a Muslim cleric prayed as a Muslim; I would expect nothing less than a spiritual leader being true to the tenets of his/her faith. If one invites somebody to pray, one probably ought to expect that person to speak to God the way s/he knows best. Why then be offended if a Christian prays as a Christian?
(If the organizers were trying to be visibly "inclusive," they might have invited pray-ers from different traditions. But I digress.)
Yes, I was startled by Warren's lead into the Lord's prayer. I had not expected it. But then I thought about it-- really thought about the words and their meanings, what it says as well as what it represents. Truthfully, it is a specifically Christian prayer only because it is contained (in slightly different versions in Matthew and Luke) in the New Testament. We as Christians believe that Jesus first taught it, so most Christians know it (though we have our variations as well). But the words, taken at face value... I hear nothing that would be off-putting to anyone coming from most faith traditions-- the monotheistic ones, at least. Perhaps, in teaching his (Jewish) disciples to pray, that's why Jesus started there?
Rick Warren is not my pastor, nor do he and I agree on some significant issues. However, given his theological bent, I thought his prayer authentic, gently reaching out to God and in God's name the best way he knows how. Seems to me that we who do not share the same doctrinal leanings might take a deep breath and back up.
If I am to live in the light of the universal (as in, offered to all) invitation of the Gospel that I say I believe, then, as the saying goes, I need to put up or shut up. Goodness-- if Melissa Etheridge can work to find common ground with him, how can I as a fellow disciple do less?