This year, Obama has laid a much stronger position for the US at the RevCon. The NPT calls on nuclear powers like the US and Russia to reduce their nuclear arsenals. Usually the US has done nothing and is the object of criticism from the smaller countries who say that the NPT is a one-sided deal, because they promise to give up their nuclear ambitions, while the US does nothing. This year we have done something by signing the Start successor agreement with Russia. In addition Obama had his huge summit conference on controlling nuclear materials. We are trying to balance the scales a little bit.
Clearly Iran will be a target of this meeting. But the US and its allies must be careful not to alienate signatory countries by appearing to apply the NPT in a discriminatory manner. We need to prove that Iran is in non-compliance. The rest of the world will probably not accept a claim that we have to sanction Iran because we don't like them.
Other countries will also perceive that we are Israel's proxy in this dispute, because Iran potentially constitutes a much greater threat to Israel than the US. Meanwhile, Israel is not an NPT signatory, while everybody believes that it has nuclear weapons, and thus is technically more of a pariah state than Iran, which is an NPT signatory and which no one believes has nuclear weapons yet.