California penal code 599(f) requires that cattle, pigs, sheep and goats who cannot walk to their own slaughter must be removed from the human food chain. The law was implemented in 2009 after an undercover investigation revealed extra cruelty at a slaughterhouse killing "spent" dairy cows.
The National Meat Association filed an injunction, arguing the state law violated the Federal Meat Inspection Act, which allows for the slaughter of non-ambulatory animals after the animals are inspected and held for observation.
A federal judge agreed with the National Meat Association, allowing the slaughter of non-ambulatory pigs in federally inspected California slaughterhouses to continue. The case was appealed and referred to the 9th district court in which all three judges sided with California (3/2010), reversing the federal judge's decision, arguing that states have long since had the legal right to decide which animals can and cannot be slaughtered, citing domestic horses as an example.
And now, the Supreme Court of the United States has decided to hear arguments for the case starting November 9, 2011. You can read the case details here.
Recognizing that I am not a lawyer, my layperson opinion is that this is not an animal welfare case. The issue of how pigs are treated is not at the heart of this case. Regardless of whether the state law is deemed in violation of federal law, non-ambulatory pigs will be killed in the same ways as ambulatory pigs. If state law stands, the pigs will have to be slaughtered off the kill floor and their flesh cannot enter the food chain. If SCOTUS sides with the National Meat Association, then non-ambulatory pigs can be set aside for observation and federal inspection, then forced onto the kill floor and their flesh sold for human consumption.
In either case, the welfare of the pigs has not changed - they are still killed, and they are still killed in the same manner. Whether they are isolated and observed or immediately slaughtered off the kill floor, their welfare has not improved in any meaningful manner.
The respondents to the petitioner (National Meat Association), which include HSUS, argue it is an issue of ethics. You can read their brief here.
I don't think this isn't even a human health issue. For example, salmonella transmission will not be reduced by not selling the flesh of downer pigs to humans. Another common zoonotic from pigs is yersinia and most pigs are asymptomatic. Toxoplasmosis won't disappear, and it won't stop swine influenza from spreading to people.
This is all about whether the state of California has the legal right to implement a law that might contradict an already existing federal law.
It is one of the few times farm animals have been considered by the Supreme Court. It will be interesting to see what transpires.