Boeing's 787 Dreamliner, a fuel-efficient airliner due for delivery in 2008, is expected to achieve 100 passenger-miles per gallon on a typical flight.
Thus, a Boeing 787 is getting fuel efficiency comparable to driving a Honda Accord or Toyota Camry with a driver and two passengers on interstate highways. For that matter, a typical bus designed to carry 50 people or so, needs to have only about 10-12 passengers to get comparable fuel efficiency. Smaller shuttles, like the vans that carry people from the Denver International Airport to ski resorts, do even better. So, this isn't a stellar achievement in theory.
The key point is that in practice, a Boeing 787 is going to be close to full on long business trips, while a sedan will rarely have more than a driver and a single passenger on a long business trip, and often will have only a single occupant.