HotAir has the link to the signing video from the Telegraph. Apparently Mr Obama was heard to ask the dean what the days date was but managed to get the year wrong.
A Westminster Abbey spokeswoman confirmed it was the president who had written the wrong date.
The White House is pleading jet lag.. http://hotair.com/archives/2011/05/24/the-obligatory-obama-thinks-its-still-2008-post
Jet lag? He was probably sleeping the entire trip.