What the U.S. did to Spain Saturday was pretty amazing. They completely dominated the Spaniards, forcing 28 turnovers and denying Spain a single fast break.  Keep in mind that the Spanish came into this game hoping for some payback from 2004. It was a nearly freakish display of athleticism and intensity, the perfect distillation of Coach K's various basketball theories: aggression, cohesion, defense and passion.
Of course, it's not the end. They have a meaningless (and therefore very dangerous) game against Germany before the medal round kicks off. They'll deal with the Aussies in the medal round, having gotten one good game from them already. Lithuania, Croatia, and Argentina will move on as well from Group A and the U.S. may see Spain, Greece, or China again from Group B.
The Aussies gave the U.S. a 10 point game in exhibition basketball, and that was without Andrew Bogut.
But given what the U.S. has shown thus far, no one other than Australia has shown that they can deal with the American defense - and Coach K has the tape of that game to study.
There are no guarantees in basketball any more than in the rest of life, but things are going well, and in fact one of the biggest opponents for the U.S. may simply be overconfidence.