Interesting commentary by Geoffrey Nunberg on Aliens and the language used to describe them. You can also listen here.

But alien still suggests strangeness and difference -- people who are "not of our sort." That's partly due to the science-fiction writers who picked the word up in the 1930's to refer to extraterrestrial beings. It's revealing that alien is far more likely to be used to describe Mexicans and Central Americans than Europeans. The tens of thousands of Irish and Poles who are in the country illegally are almost always referred to as "immigrants," not "aliens."

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As the Swiss writer Max Frisch wrote in 1965 about the European experience with immigration, "We called for a labor force, but it was human beings that came."