It just appears hypocritical for "nuclear" nations to tell other sovereign states that they can't join the club.
It's not about a "club", 189 states have signed the treaty but only five of them have nuclear weapons.
It's about countries not producing any new nuclear warheads and the countries that do have them are reducing them.
Iran is a signatory to the Nuclear Weapons Non-proliferation Treaty and the UN is convinced as is the rest of the world that Iran is producing weapons grade plutonium. That is a violation of the treaty and is a concern of every country on earth.
http://en.wikipedia.org/wiki/Nuclear_Non-Proliferation_Treaty
The treaty has three main pillars:
1 Non-proliferation- not manufacturing any new warheads.
2 Disarmament- The two largest nuclear countries, the US and Soviet Union, are actively reducing thier stockpiles. The IAEA is monitoring for compliance.
3 The right to peacfully use nuclear technology- Iran is allowed by the treaty to build and enrich plutonium for electrical power generation.
Iran does not have missle technology that could carry a nuke anywhere near US soil. The US concern is for the middle east and Isreal. If Isreal is attacked by Iran with nukes they will retaliate and all hell will break loose. However the US is the only military force that has the weaponary that can destroy Iran's heavily reinforced enrichment facilities.
The UN is the growling dog and the US is going to end up being the teeth that bite. Lets hope that diplomacy and sanctions work otherwise it's is going to get very messy.