I'm afraid we may just have to welcome the Ayatollahs to the nuclear club.
First of all, we're way too stretched out and too vulnerable in Iraq and Afghanistan to do anything. Rather than having them surrounded, we've entered their backyard and we have too many enemies there that we haven't defeated.
Second, if we were serious about nuclear non-proliferation we would've waved our big stick at North Korea, not at Iraq. We've lost all credibility in the world of weapons of mass destruction.
I agree with Hamilton's post below, this is a huge tragedy. More for the people of Iran than anything else. The people of Iran have been waging a slow cultural and democratic campaign to attempt to regain their freedom from the Ayatollahs without undoing their own identity. If the Ayatollahs get the bomb, it will most likely solidify that government's position.
Personally, I think a strike on Iran would be unjustified suicide. Iran is one of very few democracy like countries in that area. And they've got us with our pants down, vulnerable in both Iraq and Afghanistan.
But most importantly, we really need to rethink what war is. War is not a tool. It is not a means of diplomacy. It is not a means of meddling with other people's governments and decisions. War is a ghastly human tragedy that should only ever be the utmost last resort of a democratic people. War should never be voluntary. See The End of Iraq by Peter Galbraith.