Of course, Christianity is purely an import to this land as well -- a fact that is frequently overlooked. The generally pagan views of the Native American cultures are, historically, the religion that the Europeans found when they arrived and began the process of moving the indigenous people off native lands and converting them (by word and sword) to Christianity. That, as well as the fact that the Founding Fathers who are so frequently cited for a supposed support of governmental involvement in religious matters had, in many cases, left Europe to flee religious persecution. In reality, the construct of Christianity as the religious orthodoxy in the United States of America is almost purely a matter timing and power.
I don't see the argument -- and never will -- that those who aren't Christians should just shut the up and let Christians celebrate their religion publicly. I don't see it, frankly, because the flip side of that would never fly with those who are most ardent about that sort of a policy, assuming it would ever be permitted to apply consistently.
I've posed this hypothetical before, but I think it's appropriate here. Suppose that a handful of Christian families (Religious Right-ers) were entrenched in communities that over time developed a, say, Muslim majority. Better yet, a majority of the Islamo-fascists that I'm told are so prevalent in this world. For any number of reasons (convenience, economics, schools) those families don't want to move. But since the religious majority in a community is permitted to control the public displays of religious symbols and celebration, the children of these families are made to sit through Muslim prayers at school events; the families don't see nativity scenes in late December, they see tributes to Mohammed and other religious iconography -- let's say paintings of Osama bin Laden; and every official notice from the community government includes some form of tribute to Allah. Are they going to protest? I'd bet they would. If jochhejaam's view prevailed, though, they couldn't have a leg to stand on unless such a majoritarian-centered view of the Establishment Clause could somehow be limited to national majorities. That, of course, would be completely non-sensical.
More significant to me is this question: why do some Christians feel that their religion is undermined (or their faith unasserted) if the law prohibits the government from allowing public displays that are focused on only their relgious viewpoint? Is it not enough to worship God in your church and in your home and in groups (whether publicly or privately) with those who are like-minded? Why is it that the use of private means to assert religious viewpoints in a public manner are insufficient? Is it not enough for you to put the creche in your front yard each year at Christmas time? Does the vitality of your religious viewpoint depend on the government endorsing that choice and publicly displaying symbols of your religion to the exclusion of others? I've never heard anyone who supports a rollback of Establishment Clause principles adequately answer that question or any of its subparts.