BBC security correspondent Frank Gardner says the trigger for such an attack reportedly includes any confirmation that Iran was developing a nuclear weapon - which it denies.
Alternatively, our correspondent adds, a high-casualty attack on US forces in neighbouring Iraq could also trigger a bombing campaign if it were traced directly back to Tehran.
If "confirmation" of a nuclear weapons program is anything like the air tight case for Iraq, then we can expect the bombs to be raining down in Iran by the end of the year. My guess though is that it'll be a combination of "new intelligence" about an Iranian bomb and a particularly bad attack on US troops in Iraq that will convince the White House to pull both triggers.
If this were another administration, I'd say that there's a pretty good chance that this is just saber rattling, but it's not another administration. (And coincidentally, saber rattling is pretty much the last thing you want to do to a country who'd like to get nuclear weapons, because you keep threatening regime change -- especially when you're not really in any position to follow through on your belligerent rhetoric.)