With the vote for Le Pen in France and a bunch of other racist movements in Europe, this is a global issue. In fact, many of Buchanan's arguments seem cribbed from far-right propaganda that circulates in Europe. Of course, these guys tend to be a bit more tactful than Buchanan. Their (official) fear is not that the skin color of Europe will turn dark, but rather that the "national culture" of some European states is being diluted by immigrants who refuse to "assimilate".
Much of the talk about national culture is racistic nationalism enshrouded in a certain code language, and does not deserve to be taken seriously. However, the fact that even traditional European moderates find something in these parties that earns their vote means that they do tap into a real and common fear.
It is my sense that many Europeans who fear the loss of their culture are basically afraid of Americanization, which is to say, McDonaldization. In the US, where immigration is a key feature of our history, we don't really have a culture. Well, I should take that back... We do have certain cultural habits and practices which are original to the USA. All of the ones I can think of fit into the category of consumerism. This is a level on which natives and foreigners can most easily communicate and find common ground--the lowest common denominator of sorts. It forms the basis of all US self-projecting aspirations, estimates of personal worth, social intereactions, etc.
As immigration to European states increases, old-school natives find that this consumerist culture is growing stronger in their country, at the expense of what was there before. And you don't have to be some sort of a Nazi to think that this is a bad development.
Europeans, like all people, have a right to be proud of their culture, and to want to see it survive. The fact is, circumstances are making its survival quite difficult. Sure, American role models do a lot of damage, but so do foreign immigrants who don't see what the big deal is with Goethe and Schiller (for example). Then when you see that cross-cultural communication is mediated by MTV, you start realizing that even reasonable people have a right to worry.
Anyway, I think there are many good side-effects of immigration (for example, "ethnic" grocery stores keep me alive, and my foreign friends help me see global issues from different perspectives). In the USA, immigrants don't really have anything to damage, since most of our culture already operates on the MTV/Nike/Big Screen/SUV wavelength, which foreigners get just as easily as natives. However, this is not yet the case in Europe. Again, I don't want to support Buchanan's analysis, but I do think we have to face the fact that immigration significantly accelerates the descent to the lowest common cultural denominator. Anyway, my European anti-globalization buddies never seem to talk about this, even if what they protest for is in effect the same thing their enemies (the reactionary right-wingers) want: to preserve a sort of culture that is not based solely on capitalism.
Of course, when Pat Buchanan says "culture" that's code for "Christinaity". He's worried that the flow of the immigrant mob outpaces our capacity to Christianize them, and that the Church will grow weak and irrilevant, and lose its strangle-hold on US politics. This may indeed happen, and if it does, I won't mourn it. Of course, we all know that this is why Buchanan, who thinks the Church makes the best politics, is so scared of immigrants (well, in addition to his straight-up racism, I mean).
His personal stupidities aside, however, I think there is an important issue raised by his arguments. Mass immigration is really just one feature of globalization. Globalization, from a cynic's point of view, is basically the idea of making an America out of the world, of having a global melting pot where our ancestral cultures are seen as currious customs of primitive predecessors rather than living traditions. How many generations of living in the US does it take before a family's native traditions (language, customs, values, perspectives, etc.) are smoothed out into non-descript Americanism? More than one, less than four. It seems that what awaits us is a non-descript cultural Globalism, which will probably not be too different from present-day Americanism. As I said, I think Buchanan fears the globalized, culturally smoothed-out world because he doesn't like the thought that it will be run by brown people, and he's also worried that Christianity will not keep the clout that it presently has in the US. I and my anti-globalization friends don't fear either of these things; in fact, we welcome them. But that's not to say that there is nothing to fear in this global transformation.