Obviously yes. Sometimes as much as 4-5 times what you get from developing countries and more than most europian countries too. Obviously people from US spend maximum money on net, and ads just reflect that. There are more geo targetted ads on US than any other place, which results in higher payout. Some of the europian countries like UK come close, but US traffic is always good.
Some websites block developing countries. Like Veoh.com (popular video site; Alexa rank: 98) Went and blocked low monetizing countries like Malaysia, Africa, United Arab Emirates and other places.