This was proclaimed over and over again by the BBC as the reason why Arab countries - even Syria! - had turned up to Annapolis. Expect a lot more of this gibberish. In the extremely unlikely event that an Iranian nuclear weapon ever comes into being, or is even being sought, then it will be a threat to precisely one state in the world, and that state already has nuclear weapons anyway.
Furthermore, that state now contains so few people to whom even the most deranged Iranian elements might object, and is well on course to contain fewer and fewer such in the coming decades until there are quite possibly none at all by about the middle of this century, that it simply would not be worth Iran's while to launch a nuclear strike, especially given the number of Muslims, Sephardic Jews, ultra-Orthodox anti-Zionists, peaceniks, and for that matter Holocaust-deniers (let in from Russia under the Law of Return), who would inevitably be killed.
Nuclear weapons capable of reaching America from Iran? Or even of reaching Britain? Or even of reaching France or Germany? Any Iranian desire to attack any of those countries in the first place? Get a grip! Next you'll be saying that nuclear weapons can reach them from Mesopotamia (whence, again, they would have to have been launched for some unknown reason), can be deployed within 45 minutes, and can be completely undetectable by any means whatever.