This might be one of the iPad prototype?
Look like iPhone: Check
Has a home button: Check
Look like 10″ diagonal: Check
Announced docking port at the bottom: Might not be visible
Update: Source for original file: http://i985.photobucket.com/albums/ae335/unanimouswow/Prototype352.jpg
Look like the black borders are symmetrical all around. Could hide the wireless antennas quite well. But there is no docking port… so quite possibly [...]