Here's what "Chemical & Engineering News" has so far about melamine, aminopterin...

[eppur_se_muova]

Pet Food Recall
Cause Of Deaths, Illness In Pets Remains Elusive
FDA and Cornell labs find melamine but not the previously reported aminopterin in food, wheat gluten, and animal tissue

Lois R. Ember

As the scope of the U.S. pet food recall widens to include more brands and dry as well as wet varieties, the mystery of the cause of deaths in 16 pets and illness in tens of hundreds of animals thickens.

The Food & Drug Administration and Cornell University's Animal Health Diagnostic Center report finding melamine in food and wheat gluten samples. Cornell has also detected the compound in the kidneys and urine of deceased cats.

Earlier, the New York State Food Laboratory reported finding the rodent-killing compound aminopterin in samples of pet food made by Menu Foods (C&EN, April 2, page 11). Neither FDA nor Cornell has been able to detect aminopterin in samples they analyzed.
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Aminopterin is not registered as a rodenticide in the U.S., though it has been used to kill rodents in other parts of the world. It can cause kidney failure, especially in cats.
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Melamine, a small, nitrogen-containing molecule, is used as an industrial binding agent and flame retardant and, when polymerized, as a plastic to make cooking utensils and plates. In Asia, the monomer has been used as a fertilizer, but it is not registered for that use in the U.S.
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more: http://pubs.acs.org/cen/news/85/i15/8515news7.html

This is not terribly helpful, and does not contain anything really new, but it is worth seeing what makes it past the editorial process in a journal directed at chemists, and written largely by people with degrees in chemistry. So far, it seems that the contradictory test data have not been resolved.