Robert Bowling (aka fourzerotwo), the Creative Strategist at Infinity Ward, stated in the "Combat Training Experience" broadcast that there will be rewards for Prestige Mode in Modern Warfare 2, but he did not want to give away what the rewards are. I expect it will surface online soon, though. He did mention that you get to keep any accolades and titles you earn, although you will lose things like unlocked weapons and perks as you did in the first Modern Warfare.
Modern Warfare 2 Combat Training Experience