You can see how the the Fed has been primarily buying Treasuries in the 1-10 year range. According to Goldman and Morgan Stanley QE 2 will target the 5-7 year range. Morgan Stanley noted that that there are only about $550 billion of Treasuries outstanding with a remaining maturity of greater than 10 years. If the Fed really wanted to really lower longer term yields it would concentrate its buying on longer dated maturities.
The one real risk for the Fed is a rise in interest rates, which could force them to potentially sell assets at a loss. It would be politically uncomfortable for the Fed to report large losses.