Interestingly unenforceable :P
Edit: I believe it's a form of debt bondage (peonage) that is prohibited by international law too. See: http://en.wikipedia.org/wiki/Debt_bondage
Due to patents, Monsanto owns the rights to any plants with the gene--including offspring. Farmers have to continually license the plants to grow them. One unfortunate soul had a neighboring farmer's crops cross pollinate his own, and Monsanto came down hard on him--it was a pretty big incident.