Biofuel will never be a significant source of energy for the United States.
"Target" biomass crop yield: 11.2 tonnes/hectare. It's not clear what percentage of this "biomass" gets turned into biodiesel, but Wikipedia gives yields of about one tonne per hectare for higher efficiency crops. Let's divide by 5.6 instead of 11.2, just to be conservative, giving a yield of two tonnes per hectare.
Energy content of biodiesel: 37.8 GJ / tonne.
Arable land * crop yield * energy content ~= 12 billion GJ.
The USA consumes about 900 million tonnes of oil per year and about 950 million tonnes of coal per year. At 44 GJ per tone and 29 GJ per tonne respectively, that's around 67 billion GJ using numbers from 2000-ish.
So assuming our coal and oil consumption haven't grown since 2000 (unlikely) and assuming a generous ratio of biomass to biofuel conversion, the United States could generate about 18% of the power it consumes by converting 100% of its arable land to biofuel production.
Why the heck are we wasting time and money on this when nuclear power is cheap, non-polluting, and plentiful?