Like a good many professionals, Joel Makower at Two Steps Forward flies about twice a week. Unlike most, he asks the unaskable: Could air travel ever be green?
Short answer: not very, and not very soon. (And CO2 offsets aren't very credible.)
Makower has a different temperament and approaches the question in a different way than George Monbiot in Heat (relevant excerpt here), but their basic conclusion is surprisingly similar: There is no easy fix. Find another way, or don't go.