Growth of hemp, like corn or any other source, requires diverting agricultural land or clearing new land, and is ecologically damaging and raises food crop costs dramatically.
Aid agencies estimate that biodiesel-ethanol production has already driven 30 million people closer to starvation (BBC News report).

Hemp seed has 28% oil and as a crop can produce 15 gallons/acre, giving a production cost for biodiesel of $35/gallon, but using special varieties could lower the cost to $5.20/gallon.
Hemp biomass overall is a good source of ethanol, yielding 25-100 gallons/ton (higher figures depending on special varieties and some pie in the sky speculation).

Rice straw (burned in California) has been proposed as a source, but high silica content and high gathering costs make its use unlikely. Rice plants, like a number of grasses, collect silica, which is difficult to remove in refining.

All of this information is on the net, but beware of the cloud nine blogs (in the majority).