Fannie Mae and Freddie Mac, the big government-sponsored mortgage enterprises, pose a credit risk to the federal government equivalent to a AA-minus rating, according to a new report by Standard & Poor's. That makes them less risky than they were in 1991-the last time Standard & Poor's rated their risk to the government. Back then, Fannie Mae earned an A-minus and Freddie Mac an A-plus. Both agencies have substantially increased their mortgage investments in those years. But S&P said the deft use of callable debt by Fannie and Freddie controls the additional interest rate risk. Indeed, S&P concluded that Fannie and Freddie have such strong competitive positions, credit quality, and risk management that taxpayers are well-protected except in extreme situations, such as a broad-based national depression or times of hyperinflation.