Last May there was still some questions about the nature of the toxin. That appears to have been settled - at least for cats:
UC Davis researchers identify toxic chemicals in pet food - Campus NewsThe Wikipedia article on the pet food recall already includes the citation. That's fast!
... Veterinary Toxicologists at UC Davis have discovered the toxicity of the chemicals behind the deaths of approximately 16 pets in the United States this year. The pilot study conducted in April and May of 2007 found that a combination of melamine and cyanuric acid caused cats in their study to experience acute kidney failure.
The two chemicals, found in nearly 60 million packages of recalled pet food in March of 2007, have been added as a source of protein in some brands of pet food, but until recently had not been tested for their toxicity.
"There were no published reports of toxicity studies examining the combined effects of melamine and cyanuric acid in any animal species," said director of the study and associate professor of Veterinary Toxicology Birgit Puschner. "We needed to determine with certainty whether or not melamine or cyanuric acid alone or in combination, could cause renal disease."
Although the University of Iowa conducted a similar study with pigs, UC Davis is the only research institution to find and publish the cause of toxicity in the recalled pet food. Their findings were published in the November issue of the Journal of Veterinary Diagnostic Investigation...
My personal sense is that Americans have mostly forgotten about the problem and have not changed their buying habits.
For example, Eukanuba, who used to make our dog's food, once boasted of their US based supply chain. They are now owned by Iams, who's name is now on our food. They make no such claims.
Clearly consumers have not been demanding any real changes.
It's hard to be a "market of one".