Well the US and Israel can't just blast Tehran with nuclear weapons or even with conventional bombs until all other options are exhausted. We're making history here that will last for hundreds of years, and we don't want to go down in history as a nation that bombed Iran when a diplomatic solution was still likely to occur. I don't see the US using nuclear weapons in any scenario. We can take out their nuclear facilities with multiple bombing runs using GPS-guided bunker busters. Israel doesn't have our air power, so most likely if this ends in a military strike we would bomb Iran to make sure Israel isn't forced into using nukes.
But Iran will not go unchecked. I think the patience being displayed by Bush, Blair, and other western leaders is a sign of very high confidence that Iran is not close to building working nuclear weapons (and I hope they're right about that).