If the seed is patented by Monsanto, you can't keep it to replant. That's how Monsanto sucks in farmers-they are FORCED to buy seed every year, rather than keep back a portion to replant as they normally would.
For those of you that don't believe something like this has happened, or could happen, GOOGLE is your friend. There have been many cases of farmers being sued for patent infringement and MONSANTO wins them all, or they settle out of court because the farmers can't pay their legal bills.
They DO hold farmers responsible for any seeds that are a product of cross-pollination by neighboring farms. It's a sad and scary world out there, and Monsanto is seeking control by way of our food supply. And there is definitely a desire to push small family farming operations out of production.
If you don't think it's a big deal, I URGE YOU to do some research. You will find that this isn't some "tinfoil hat" conspiracy theory, but that it is a real issue, and the only way to resolve the issue is to inform all consumers of the ugly truth and hope that enough of us are compelled to do something about it. Food is big business, although you can certainly choose to be the Ostrich...
Good read: The Omnivore's Dilemma
Excellent Documentary: Food, Inc.