The dear wife and I took a 737-800 to Phoenix en route to Orange County over the weekend. If you haven't taken a Southwest flight recently, they're flying many new ~2 year old 737s, which really enhance the flying experience compared with older planes. While I'll go into this more later, I just wanted to note how fuel efficient flying is these days. From here, I find that Southwest's new "737-800 burns 4.88 US gallons (18.5 L) of fuel per seat per hour." Since the plane's cruising speed is 514 mph, that means each seat (or person) gets ~105 miles per gallon when flying on this plane (i.e. 514/4.88). Not too shabby for one person whose contemplating driving over taking a plane. However, if four people are piling into a Prius instead of driving, the calculus changes. Suddenly, the flight is about 26 miles per gallon for the group (105/4) and the car would be around 50 miles per gallon.
Note also that aircraft emissions (due to their altitude) affect the climate more than equivalent emissions from vehicles, although it's hard to nail down by how much.