They don't have to bump the physical album release date, but can still bump the itunes download to ASAP!
I think they should put everything on Itunes along with Fast Cars (and scramble/decode it so it can't be burned) as a free bonus to anyone buying the entire album!
This way all the excitement will cause people to download and get the extra track.
Hell, they probably have another b-side they could offer as a bonus. Something unheard of as of yet. That would blast itunes sales into the stratosphere!