On the fifth anniversary of the September 11 attacks, Pastor Tim Keller was asked to address to deliver the sermon at an ecumenical prayer and remembrance service. The NY Times report of the event can be found here, but I could not find the sermon available at the Redeemer Pres website.
Update: Thanks to the worship.com folks for pointing out to me that the text of the message is available on Pastor Keller’s son website. See this post for a link.