Incorrect. Monsanto seed did NOT drift onto Bowman's land without his knowledge or consent. Thus far, no such case has been litigated where seed drift or cross-pollination has occurred. Obviously that is a very big question that will come out of this ruling, should the court find for Monsanto, as it will arguably put the onus of burden on farmers to test for and destroy infringing crops caused by cross pollination. That issue, however, is not in debate here.
Bowman realized that a staggering percentage of soybean seeds on the commodity market were Roundup-Ready GM seeds. Normally a farmer has to sign a contract that he will not replant any additional seeds and will buy future generations of seeds from Monsanto. Replanting seeds for these farmers has not been considered a patent infringement but instead a contract violation. The patent infringement idea was unprecedented until this case. Bowman, who had not signed a Monsanto contract, simply decided to buy contract-free seeds on the commodity market, as Monsanto-contracted growers can sell the seeds they are not allowed to replant for general purposes such as food production. Bowman had the novel idea to take these seeds and plant them, spray the seeds with Roundup (thus killing off all the non-Roundup-Ready seeds), and have contract-free Roundup-ready seeds that he could replant at will. Monsanto, which monitors the purchase of Roundup to Roundup-ready seeds under contract, determined Bowman had purchased enough Roundup to be running an un-contracted operation. Unable to ping him on the contract issue, they requested him to stop. He refused, and they sued under the patent infringement theory.
The question that will be debated here is whether or not subsequent generations of Roundup-Ready crops, by the act of growing them, independently constitute patent infringement. Normally for infringement to occur there has to be some performative action. Monsanto is arguing (and the lower court agreed) that the performative act of planting the seeds in the first place is sufficient to transfer infringement to subsequent generations, and therefore the plants can essentially infringe upon each new growth without Bowman's performative action on subsequent growings.
It may seem pretty dumb, but it has the potential to majorly impact the food industry. If the court finds for Monsanto, the "auto-infringing crop" theory would make accidental infringers of any farmer who encountered cross-pollination or seed drift. Although no such cross-pollination has been successfully argued -- in all cases where farmers have brought this defense, it has been very well proven that they were lying through their teeth and had planted Monsanto crops in violation of their contract. Conversely, if the court finds for Bowman, this would in effect nullify Monsanto's patent protection on their seeds, as no farmer would buy from the developer, bound to a contract, where they could just go out and buy commodity seed at a fraction of the cost.
I've researched GMO patent intensively, written articles, and have followed the case for a while now. I think the one constant among GMO patent cases is that both sides -- Monsanto and farmers alike -- have done nothing but provide a tremendous amount of misinformation about the other side. No party line can be trusted. Monsanto argues that they're just trying to make a living and don't gouge anyone, being a humble food producer. Farmers argue that they're being put upon by the big corporate food monopoly and haven't done anything inappropriate other than try to grow organic foods. Both sides are lying and are trying to wage war to maximize their profits. Being as rabidly anti-DRM as we are, I suppose Slashdot readers will support the farmers. Either way, this issue is pretty big for determining whether the judiciary is embracing the pendulum swinging back to more restrictive patents or is continuing the trend of expansive patent protections.
Also worth noting, Monsanto's patents on Roundup-Ready soybeans are set to expire in the next few years, IIRC. The question is going to be entirely academic and legal and will likely have no effect on Roundup-ready crops at all after the patent expires.