Still trying to get a grip on biofuels. I came across this assessment of their potential from WorldWatch:

In the most optimistic scenarios, bioenergy could provide for more than two times the current global energy demand, without competing with food production, forest protection efforts, and biodiversity. In the least favorable scenarios, however, bioenergy could supply only a fraction of current energy use by 2050, perhaps even less than it provides today.

There you have it.