Hello and sorry for my English.
I just want to advise that it could be the same problem of my Arduino NG: the reset button is not working. Then I am only able to update just during the 6 seconds after plug my board to usb (because I am not able to reset the board). (I don't have check yet what is the wrong connection in my board because it has arrived one day ago, but at the moment it's ok re-plug every time the board - it is a 3 second task)
Sorry if it is a stupid idea for your problem, but just in case.