The number I was using last year was 20%, so I was estimating this year would be around 25%, before I realized there were a variety of numbers "out there". One of the commenters on Robert's post uses 20%.

Gail, I feel your pain! Comparing different data sources is a perilous undertaking.

Here's another citation:
Ethanol to take 30 pct of U.S. corn crop in 2012: GAO
"About 27 percent of this year's corn crop will be used to make ethanol, according to the Agriculture Department."

Unfortunately I don't have the Ag Dept stats themselves.