BUTANOL...Dupont is currently in final development of the enzymes, and pilot plants are going up in conjunction with BP Amoco.

Currently, we're stuck with ethanol, since we don't have any strains of microbes than can ferment corn into butanol without dying off too early in the process. Dupont is developing microbe strains that are more resistant to butanol...which packs a lot higher energy density than ethanol, has a freezing point much closer to conventional jet fuel, and can go through existing infrastructure without corroding it (unlike ethanol).

Read last month's issue in PopSci...the cover story is on alt fuels for aviation, and a prototype hydrogen powered ramjet designed by the concorde folks.

We can get hydrogen and butanol with current tech, and it's possible to run jets off of those fuels...in conjunction with continuous descent procedures, lighter and lighter materials, and better energy storage devices (some Chinese dudes are doing stuff with torric coil batteries that could potentially charge up off the grid (nuke, etc.) and power N1 fans like a mofo))...we might be ok for another 50-100 years.

Of course, we could also do some pretty obvious things to save fuel like using tugs to get planes much further down the taxi line before starting engines, and shutting down the second you're clear of the hold short line and having tugs bring you in. And having computer assigned taxi clearances based on algorithms that compute the most efficient sequencing based on aircraft position and requested start time.