When Standard & Poor’s downgraded the U.S. government’s credit rating in August, many predicted the worst. But nearly a year later, mortgage rates have hit record lows, the government’s borrowing costs have eased, the benchmark S&P stock index is up, and investor enthusiasm for Treasury debt has picked up. “In Omaha, the U.S. is still triple-A,” said Warren Buffett in 2011. “In fact, if there were a quadruple-A rating, I’d give the U.S. that.” Last month, S&P said that political and fiscal risks could lead to a second downgrade of U.S. debt by 2014. Moody’s and Fitch Ratings have kept their top grades on the U.S., though, both have a negative outlook.