In his own deft way, Ahmadinejad is calling the bluff of the West by pretending that none of the threats of war from France and the U.S. are happening at all. You may remember that the French Foreign Minister claimed that the only way to eliminate Iran's nuclear threat would be war if strong sanctions weren't imposed. That's not the same as calling for an outright war, but in diplomat-speak the mere mention of the term "war" is significant. It is a clear first step to the real thing, and not to be taken lightly.
Another significant point is that the threat came from France. Such verbal bullying we have come to expect from a cowboy President who had never been to Europe before his election. Not from France, who is not known for such things. France is not thought to be in goose-step with the Bush Administration either. So it shows there is a real coalition building in the West for war with Iran. That's serious.
Ahmadinejad's nonchalant rhetoric is probably aimed at trying to calm his population down and improve their self-esteem by another laughing round at the West. Thumbing your nose at the big guy always has a great feeling to it. He's also throwing the fear back at the West. Now the West has to react to its own words since Iran isn't giving us any pushback. Let's see if the mention of war continues.