The most important motivation of Richard Branson to develop biofuels is not the environment, but the peak in oil production which will probably happen before 2015 (according to Shell). After peak oil the growth of aviation will be limited by the availability of energy, but more likely by the oil price explosion if the difference in demand and supply start to grow.
Agree that biofuels are a very dubious 'alternative', but I admire anyone who is trying to do something constructive (and risk $400 m of investment).
For the interested, a study from the ASPO about the relation peak oil-aviation (27 pages, pdf):
http://www.aspousa.org/proceedings/h...s_10-19-07.pdf