I'm not buying it. I don't believe the average American supports terrorism at all. I have simply talked in person to too many Muslims, chatted on line in this kind of forum and in chat rooms with too many Muslims, worked and played with too many Muslims, and watched too many prominent Muslim interviews on television to believe that Muslims widely condemn terrorist acts committed by Muslims. Lack of support for that is tepid and non committal and non forceful. Probably that is mostly due to the fact that they fear other Muslims if they speak out forcefully.
Again most Muslims are not openly pro-terrorism. But most Muslims also don't openly oppose it. Even our USMB Muslim members don't deny that the Qu'ran intends that the whole world will be subject to Allah. And they don't deny that the Qu'ran condones killing any infidels (i.e. non-believers) as necessary to accomplish that.
I think you need to consider that there are about 1.5 billion Muslims in the world. So even though you might read of millions of Muslim's support for terrorism, that is a very small percentage.
There are plenty of Muslims that condemn terrorism. It's just their voices are drowned out by the growing anti-Muslim feeling.
A large segment of the Muslim world supports the Palestinians in their struggles against Israel. That support is often framed as a support for terrorism.
Again, I don't doubt that the average Muslim in most places would say "no" if asked if he or she supported terrorism. It is a different story, however, if the question is whether they denounce and oppose a specific terrorist act or specific terrorism in a specific circumstance. Those who will stand up and denounce that are not that easy to find.
As to when it will be time for a mosque near ground zero, that time will be when most Muslims do denounce terrorist acts committed by Muslims (and everybody else) and we no longer have lists of Muslim terrorist acts twenty or thirty times longer than that committed by any other identifiable group.