Let me start by saying that it was irresponsible for someone of Tancredo's position to even hint at such a thing. Leave that sort of rhetoric to those of us Islamofascist-haters who don't actually represent the interests of the United States government.
I couldn't find the post in the archives, but I know it is there, where I talked about this very idea. What Tancredo suggested should be threatened, but never executed. Why? Because we aren't at war with Islam, we are at war with Islamofascists. There is a difference, however subtle.
Hugh Hewitt asks what the strategic value of this idea is...if you let it be known that bombing Mecca is a potential response, perhaps that would motivate the "moderates" in Islam to take more productive action against the Islamofascists. The moderates fear reprisal from the Islamofascists. They will not take significant action against them until it is in their interests to do so. That, my friends, is strategic value.
I still think that Tancredo is an idiot for having said it though.
7/21 Update
Found it! Courtesy of someone who came over from the Jawa Report where I linked the piece when I originally wrote it...Mark's Remarks
I agree that Tancredo was a total friggin buffoon for saying such a thing. Whether or not it should be policy, however, is a different matter. Perhaps putting pressure on the moderates in Islam by saying, "you know, it is nothing personal, but if you don't do something about the scum perverting your religion....well, ahem" could get more people on board the anti-islamofascist bandwagon. I guess in some ways strategically it is setting up a "if you get us, we will get you" scenario to keep the islamofascists thinking about if we bomb the US, goodbye hajj, unless we want to be glowing for a few hundred years.