Okay, did some research and reporting into Bush's statement that Iran must be denied not nuclear weapons, but the "knowledge necessary to make a nuclear weapon." This isn't an entirely new position from the White House, but it had kind of gone missing from administration rhetoric, so it's return to prominence is potentially significant. Specifically, I'm told that the crux of the matter is that there's no evidence of an active Iranian nuclear weapons program. There is, however, a uranium enrichment program that could at some point be used as part of a weapons program.
But basically were you to want to use military force against the Iranian nuclear weapons program tomorrow, you'd run into the problem that there's nothing there. If you define the threshold down to some kind of war on knowledge, however, you put yourself in a position where maybe you can define the centrifuges Iran already has as constituting the knowledge they must be denied or at least a program to obtain the knowledge. Thus you have, on the level of rhetoric though not international law or sound diplomacy, the justification for military action.
On the other hand, perhaps Bush just screwed up and doesn't know what he's talking about and there's nothing to worry about. Alternatively, maybe he knows exactly what he's talking about and we ought to worry. Or maybe we ought to worry that he doesn't know what he's talking about. At a minimum, I'm kind of worried.