Corn seeds are not wind blown, the pollen is. It is a fact that GMO <> Non-GMO corn cross pollination happens, and when that does your seeds become tainted. To collect and replant those seeds is patent infringement. Wikipedia has a lot of information about Monsanto court cases.
Monsanto actually has a page "busting" the myth, but I'm going to link to it because it clearly states that monsanto has the right to enforce patent fees in this case.
http://www.monsanto.com/newsviews/pages/gm-seed-accidentally...
Here is the quote:
Monsanto has a long-standing public commitment that “it has never been, nor will it be, Monsanto’s policy to exercise its patent rights where trace amounts of our patented seeds or traits are present in a farmer’s fields as a result of inadvertent means.”
So they are saying that, yes they own those patent rights, but no they won't enforce them. It's pretty clear cut that one has a legal obligation to pay monsanto, but they're being nice by not enforcing their rights. ( If you watch Food Inc, you'll see how nice they are about non-enforcement in reality ).