That's a darned good question, says the BBC's security correspondent Gordon Corera. The question is bubbling in part because a major announcement is expected to coincide with the anniversary of the Iranian revolution.
Unfortunately, because the answer isn't known, it could lead to an "accidental war."
If Iran was able to run a parallel, second enrichment program which it had managed to keep secret, then many of the estimates of how far Iran was from mastering the technology might be way of the mark. But this remains an unknown.
The degree of uncertainty can cut the debate over action against Iran in both directions.
Some voices argue that Iran remains at least five years away from nuclear weapons capability, and US intelligence estimates have consistently pushed back when that might be - so some argue there is no rush.
Other hawkish and pessimistic voices argue that Iran could soon master the technology and the time-frame for action lies this year.
Israel is keen to emphasise that it sees the shorter time-frame as the valid one and is willing to take action.
The US has been playing down its willingness to engage in military action but is currently pushing the Europeans to squeeze Iran financially.
'Accidental war'
But conflict between the US and Iran is still possible.
President Ahmadinejad is facing his own domestic problems with mounting criticism of not just his approach to foreign policy and the nuclear issue but also his failure to deal with economic concerns at home.
This could lead to other power centres in Iran forcing him to back down but could also encourage him to take a harder line on the nuclear programme in order to try and rally support.
At the same time, Washington has been increasing the pressure over Iran's alleged involvement in Iraq.
With US troops so close to Iran's borders, a small event could easily ignite a wider escalation and even trigger an "accidental" war - although conspiracy theorists might argue that there are some in both Tehran and Washington who would like to engineer just such a confrontation and blame the other side.