GF was travelling recently and the captain came on and announced that her plain was burning 2 gallons of fuel per mile. She was surprised - it didn't sound all that bad since there were 100s of people on the plane. I though I would find out for sure, so I looked it up and found this:
http://www.iata.org/whatwedo/environmen ... ciency.htm
Highlight: 49 mpg per passenger in 1998, new aircraft as high as 80 mpg per passenger. That's a reasonably favourable comparison versus driving, even for a couple, though driving might still beat this for, say, a family of four in one car.
Just for laughs I though I'd compare this to taking the train, and found this:
http://www.chooseclimate.org/climatetrain/ecobal.html
Highlights: trains are in the 100-200 mpg range (per passenger), while aircraft have 3x greenhouse impact per unit of fuel used verus ground transport.