His campaign said that he "misunderstood the definition of decriminalization". Now, he's an attorney, allegedly taught law (constitutional law at that), if I recollect as an adjunct professor, and is a U.S. senator. As such, he is a "wordsmith", who had God damned well fucking-A better know definitions of simple words and legal concepts! Thusly, I call BULLSHIT. He said what he did in order to garner votes from the most people without jeopardizing the whole thing.
Do your research. Obama selected Biden, the person who coined the term Drug Czar, as his running mate. He cannot simply choose which laws shall be enforced and which won't. The ONLY difference between Obama and McCain is that McCain is certain he will will continue the War on Drugs, whereas Obama doesn't seem to know the definition of decriminalization.