USAF works on project which is called by "Blackswift". They design SR-72 that it will be able to fly with "5 Mach" speed at high altitude.
The long and the short of it is that no any AA missile will be able to shoot SR-72 at this altitude with that speed. However with that speed plane can't chage to direction easly, so air route can be predictable.
http://www.janes.com/news/defence/air/jdw/jdw080310_1_n.shtml