I'm not sure it was really a "mess up" by the US in Iraq, all they had to do was open up for the inspectors required by the UN. I can see this going the same way in Iran to a point. The US will go to the table and the same demand for inspectors will be made, if inspections are not allowed by Iran then we will be right back where we are now. The only difference would be the extra time obtained by Iran to finish their work on a weapon, there is no way anyone could attack Iran while talks were on going.
True however you have to understand that Iraq back then knew that US was wrong in their assumptions.
However the US pushed trough and years after the invasion the whole world could see that the US and its mountain of paper work and evidence was worth nothing.
Also wikileaks revealed enough info to hurt the US credibility for a long time to come.
Thinking, believing and saying that a person is a murderer does not directly mean that a person actually is.....
Iraq was a clear case of this.
Iran is in a similar situation, again i am not saying i believe Iran as i do not however lets face it.
Besides some indications and indirect evidence there is nothing serious that proves beyond a doubt that Iran does build a bomb.
Because IF this proof is available then why is the US and the west not coming forward with it?
And you have to understand this that N-Korea could not hide that they build a bomb and they happen to be the most closest off and inaccessible nation on the world.
And the thing is NK could not hide it despite their best efforts......
If Iran is building a bomb or is close to it then it will come out as every agency and satellite have their eyes fixed on them.
And still no direct proof.
This is exactly the reason why the world has such a hard time supporting more and heavier sanctions and why the world has serious troubles in direct action.
Because after Iraq the world has become wary of the term: "Grave concerns"
Ok lets say this i do not trust Iran and personally speaking i do not have any reason to think that they are telling the truth, however how is the US going to explain to the world after a invasion (Assuming it has come to the point for hard and direct actions) IF this whole nuke program was peace full after all?
It will lose its credibility and its foot print in the middle east.
This loss has such a huge impact on the future for the west that both Israel, US and the EU does not want to go there.
Iran knows this regardless if they build a bomb or not.
If Iran allowes inspectors it would not change much the US will not accept final proof that the program is actually peacefull, because as international analysts have said ot would put the US into a bad bad situation...
So either the West does proof Iran has a bomb......or Iran shows them the proof.
Still either way Iran will play it to the last moment so that the blow to the diplomatic relations and international community will hit like a psychological nuke.
But then again thats only IF they tell the truth.
And i am pretty sure that IF the US knows that Iran does not have a bomb it will do whatever it can to take Iran out because the US does hopefully see what is going to happen if Iran happens to speak the truth afterall....
And thats what i call a "Grave Concern"
Anyway i might explain it wrong here but i am sure that the people here on the forum could explain it better.
Here a wiki link (I know its not always accurate but its a good read

[ame="http://en.wikipedia.org/wiki/Iran_and_weapons_of_mass_destruction"]Iran and weapons of mass destruction - Wikipedia, the free encyclopedia[/ame]
The whole thing that gives the US the power to accuse Iran is that Iran refuses to work with the Atomic agency but that still does not mean Iran is wrong.