I’ve discovered that typically, a farmer who grows the forbidden fruits and vegetables on corn acreage not only has to give up his subsidy for the year on that acreage, he is also penalized the market value of the illicit crop, and runs the risk that those acres will be permanently ineligible for any subsidies in the future. (The penalties apply only to fruits and vegetables — if the farmer decides to grow another commodity crop, or even nothing at all, there’s no problem.)
You can access the 2002 Farm Bill on the USDA's website here, and Title I is the "Commodities" section Mr. Hardin is referring to. For a more in-depth look at this section, you would have download the 2002 Farm Bill itself and look under pages 11-23.
Another good source for information from the USDA is the publication "Eliminating Fruit and Vegetable Planting Restrictions: How Would Markets Be Affected?"
Here is a pull-out from the "Discussion and Implications" section of this publication:
Land Is a Minor Constraint for Many Farms
About half of the area devoted to fruit and vegetables is grown on farms
that certify their acreage with the FSA and therefore are likely to receive
program payments. Farm program rules permit these farmers to plant fruit
and vegetables under certain conditions. A farmer can plant fruit and vegetables
on the portion of his or her cropland that is not base acreage without a
reduction in payments. If nonbase cropland is not available, the farmer can
lease or purchase nonbase cropland and reconstitute the farm to include the
new acreage, again without incurring a payment reduction.
Farm program rules permit fruit and vegetables to be produced on base
acreage if the farm has a history of planting fruit and vegetables, but in
these cases, payments are reduced on an acre-for-acre basis. In 2003 and in
2004, payments on over 600,000 acres were forgone in order to plant fruit
and vegetables on base acreage. Thus, nearly 5 percent of fruit and
vegetable production was on base acreage. On average, these farms gave
up payments of about $22 per acre.
For farms that do not have base acreage—farms that are likely to be primarily
fruit or vegetable farms or livestock farms—planting fruit and vegetables
is not restricted. These farms can expand their production based on
land availability and expected market returns.