This has been a concern for a while. I hope it doesn't happen and i think the market will be what can prevent it. Nobody wants Monsanto weed so I think they would struggle to sell their product. The danger comes from over regulation of the market that would funnel business to them artificially; a law that requires seeds to come from an "approved distributor", for example.

This does worry me a bit but if it does happen, the market will likely just move back "underground" and all the producers and consumers go back to being outlaws like in the old days.