The only sustainable biofuel is used chip fat, concludes George Monbiot in his latest Guardian column. While I agree with him on most things, I wouldn't go quite as far as that (yet). While not all biofuel programs based on agricultural waste are truly green (he cites an example of waste that would normally be scattered on the fields and help to avoid erosion), there are lots of different kinds of agricultural waste in different parts of the world, and I am sure that for some of them it makes perfectly green sense to turn them into biofuel. Such as the bagasse, a waste product of sugar cane, which the Cubans are turning into ethanol. Or wood chips in Scandinavia.
I think these need to be considered one by one, even though the one green option that is available to all people in all places, is to use less fuel. But that, of course, runs counter to the economic growth that we are so addicted to.