Don' forget the simple stuff:
cold ambient causes the tire pressure to drop ~ 1 psi for every 10 degF drop -> lower psi = lower mpg
colder air is more dense, therefore the computer adjusts by putting more fuel in the charge -> more fuel per cycle = lower mpg (maybe, not sure, but it also means more HP!)
i think these cars run a little rich by default until the car warms up, and when it's cold it takes longer to get to operating temp.
They always run a little rich at idle, and the car uses more fuel to idle whan it is cold (see above re: density). that time at 0 mph is a real heavy weight on mpg calcs
EDIT: Wow, I'm really the master of what everbody else said. Maybe one day I'll have something original to post.
