This was figured out by FlyerTalk last year and worked for about 3 months before the Mint caught on and changed the terms of credit purchase to be a cash advance, which doesn't qualify for rewards and carries additional interest terms. That said, it was pretty clever and awesome while it worked.
WSJ article: "US Helps Frequent Fliers Make A Mint" http://online.wsj.com/article/SB126014168569179245.html