Part of the change in American tone must certainly be attributed to a new US President, US Attorney General, drug czar, etc.. Also, Mexican President Felipe Calderon has earned many chits with the US for his aggressive attacks on drug cartels and even (some) corruption in his own administration. Maybe folks are cutting him more slack for that.
Or maybe with the body count piling up in Mexico (and the nation's pathetic prisons witnessing so many escapes they may as well install revolving doors), other priorities seem more important now than whether Mexicans can possess up to 5 grams of pot or a hit of LSD without facing criminal penalty.
Do those factors explain the American reaction or are there other reasons Mexico's decriminalization of low levels of drugs had been greeted in the United States with yawns instead of concerted disapprobation?
UPDATE: Via Drug War Rant, here's an explanation of the new law from the Tonight Show's Conan O'Brien: