From what my friends say, the terrorists who would be carrying out attacks in the US are all busy in Iraq trying to expel the Americans.
If Israel were to attack Iran, there would be a stream of newly recruited terrorists heading towards Israel, not immediately towards the US. There would be a rise in terrorism in the area, in Iraq as well.
There would be an increased threat in the US, but homeland security would see it right away. Plots against the US take a lot more time to organize and get underway, so we would have some warning something was going on.
The question is what would we see? Al-Q thinks big. Maybe they'd target oil refineries, but I understand that's not how they operate, they're still thinking big, as NYC big. But they're not the only ones around; my question is how long until we see suicide bombers walking into synagogues in the US? It doesn't take much planning, it doesn't take many resources. So far Al-Q has been the focus, but when will other groups decide that nobody is doing anything and take matters into their own hands and just start doing random acts of terrorism?