"American Hiroshima" has nothing to do with a dirty bomb, IMHO. A dirty bomb is too unwieldly, too uncertain as to the amount of damage/contamination could occur. "Hirsohima" took many more lives that an dirty bomb could.
With Iran so close to getting the material to build its own atomic weaponry, and given their patience, I don't see a nuclear terrorist event occuring on our soil at this time. It would be much less trouble for Iran to simply build a suitcase nuke and give it to bin Laden for the proxy destruction of an American city.
I would go out on a limb and say that any Al Qaeda attacks on American soil at this point would be of the conventional variety, but perhaps with a Chem or Bio component (which would of course make it non-conventional). Specifically, I'm guessing we're going to see our first overt AQ suicide bombings on American soil. And it's going to happen quickly, and preferably (for Iran) before any conflicts between Israel and Iran. It may even be used as the lynchpin to start a Mid-East war - e.g. a quasi-Archduke Ferdinand scenario where AQ attacks the U.S. and the U.S. decides to retaliate against Iran...
Don't doubt for one second that AQ, Iran, Syria, and Hezbollah aren't in concert here. They're brewing the perfect storm, and with Hamas taking over in Palestinian territories, the cake might be ready to bake.
I think the cake is ready to eat. All it would take is one little match and everything could go up in flames. As a poster yesterday noted, terror is congealing into states. Hamastan, Iran, Syria, even Venezuela and Mexico. The blood in the streets is going to happen. Only a question of when and where.