Read Big Al's letter to Americans on CNN here
The context of this particular verse (28:67-28, Sura "Al-Qasas," or The Narration), is very clear. It follows a graphic description of destruction and devastation that will befall those who fail to repent of their injustice.
It also sets out the terms of the tradition Muslim warning to the enemies of Allah. "And never will your Lord destroy the towns until He sends to their mother town a Messenger reciting to them Our Verses." This is precisely what Ahmadinejad is doing in his letter.
Dump Bush, allow the Muslims to destroy Israel, and adopt Islam — or else you will be destroyed. This is Ahmadinejad's message.