The problem is not world opinion as such, but that the American people need to be prepared for the consequences of an unprovoked attack on Iran. The President would need to start the process of educating Americans that Iran is a threat to our survival. This would be impossible at this time given the almost total lack of support (and outright hatred) for Bush among the elites. Bush does not have Reagan's ability to speak directly to the American people over the heads of these elites (unfortunately). Therefore, as you indicate, some horrendous event will be needed to do the job of educating.
I agree that world opinion as such is not an issue. But if we use nukes first -- for any reason -- we will see an explosion of hatred against us that will make what we experience today seem like an Easter egg hunt. I'm talking a total global economic embargo, UN resolutions branding us a rogue state, maybe even "quarantine actions" by foreign forces. The new treaty with India will go up in smoke. Russia and China will completely freak out and kick off an arms race of 1914 proportions. No more cheap Wal-Mart crap from China; no more cheap labor in Malaysia and Indonesia; even the Japanese might recoil in horror. Venezuela, Saudi Arabia, and the Gulf states will likely cut us off. The stock market could very likely collapse, creating a deflationary slide. It'll be bad.
We'll eventually have autarky here -- either by choice or because no one else in the world will trade with us. I'm all for choice, myself. It sure beats the alternative.