One of the reasons I enjoy responding to interesting points that make me examine either my own, or someone else's, logic is it actually sometimes causes me to realize or hit upon things I had not really seen before. Yours did that actually with touching on the notion of White Privledge, and how that logic lends itself to an argument of the U.S. potentially being a Christian Nation.
White Privlege basically suggests that the very essence of being "white" is in and of itself a privledge due to the way society works. That when a person is being described that it's assumed they're white, unless said otherwise. That those we see in various forms of media by and large are white by default unless an effort to "diversify" is actively made. That due to the institutionalized barriers to other races over the years, on average a white individiual is better off from day one than a minority individual is likely to be in terms of wealth, location, educational oppertunities, and on and on. That essentially without any actual purposeful action or even intent, so much of it is ingrained as default in our culture that it provides a "privledge".
The same kind of logic in sorts could go to the notion of a "Christian Nation". You bring up the religious holidays that have no basically become "secular", celebrated by people across all stripes. But that could be argued that regardless of how they may be celebrated now or their loss of religious meanings, the fact that said religions "holidays" are still essentially viewed as the "default" and are widely celebrated, even outside of that religion, is a point in that religions favor. That essentially the spread of christian holidays to non-christians, even with the loss of the christian meanings, is actually an instance of christianity infiltrating the society and culture at an even greater mark. You look at what you did in regards to me, and the assumption that I'm christian. It's the default in this country by and large. Unless you're told someone is athiests or jewish or muslim the assumption typically is that they're christian. You hear the media talk about the potential "issues" someone may have due to being islamic and running for office, or even some of the "christian" sects that aren't viewed as christian like Mormonism (and all the talk surrounding Romney with that). Yet rarely do we hear talk about how a politicials religion will hurt them if that religion is "Christian". While it's intensity of its impact, and specifically it's religious impact, may be less significant it could be argued that the spread of said impact is just as large as it's ever been.
It's still a slowly forming thought in my mind, and given that I don't buy the white privledge logic to the degree that some do I don't know if it's an argument I ultimately would subscribe to myself, but coming at it from a devil's advocate stand point it's beginning to form an interesting platform of an argument in my head.