I'm assuming that if you haven't been living under a rock offline all night, you're aware that Iran can theoretically have a weapon in time for May Day.
The world is fundamentally different than the 18th century. Isolationism makes no sense when it is feasible to pay a hundred men to effect an operation that will put a dozen or two on the sharp end of a spear capable of infliciting megadeath. Ahmadinejad has threatened Israel's existence, not that I give a rat's ass about Israel right now, since they're still betraying us and selling our secrets to the PLA), and openly funds the people who talk, in public, about invading the west from within in order to force sharia (spit) upon the world.
We won the Cold War. We made messes in the process. It's time to clean up the messes -- now. Much as I'd rather focus on cleaning up domestic trouble, there are mad dogs in the street: we do not have the luxury of allowing enough time to pass for Iran and any other descendents of AQ Khan to gain plausible deniability for an attack.
UPDATE: and now it gets curiouser. Comments over on WoC (link in sidebar) suggest that Natanz may not already possess that many centrifuges, but will need to build them. In which case... once again, only the players know the score, and the rest of us are wondering whether it's all-clear and the Iranians are playing their traditional two-step, or whether we're already on the express-elevator.