We don't actually have full statistics because the government doesn't keep track of them. The figures from the USDA, which are based on estimated plantings, were that 43.7% of the corn area was for GM varieties, but if you look at the variety registration for this year, about three-quarters of the varieties that will be on the market are GM corn varieties, meaning that in many cases the genetics that producers will want will only be available if you buy a GM variety.
On December 3rd, 2009. See this statement in context.