I personally doubt they're going to really build an SR-72, and if they do it won't be what people expect (it won't be a spyplane, it'll basically be a testbed for supersonic testing).
From what I recall reading, what they're actually wanting out of this is basically a more direct ICBM-esque weapon. ICBM's trajectory takes time and it allows some possible intercept. Being able to do more direct paths means less time for detection/tracking and response. It doesn't have to be nuclear either, they want super speed missiles to effectively replace cruise missiles, where the speed offers extra energy and piercing power (bunker busting) capability.