In what way is it a mistake, Brod? The USA had no choice - it is OBLIGED to issue a visa to foreign leaders. This is a non-story. The only interesting aspect about it is that Ahmadinejad is coming to the UN - it'll be fascinating to hear what he has to say. My own recommendation would be for him to stick to talking about the obvious illegality and political nature of the UN sanctions.
Second rocket fired from Gaza explodes inside Strip (Haaretz)
from the article: U.S. grants Ahmadinejad visa for address to UN Security Council