I looked up Jerry Pournelle’s site and he was worrying about the US getting overwhelmed with immigrants. To start with, it looks like he’s using scare numbers, 100 million US immigrants in the next 20 years. The US has recently naturalized 1/2 million to a million immigrants a year (pdf). The base value is about a 1/2 million with the figure doubling during amnesty periods.
So how high was immigration into the US in the past? I recall the last big wave was in the early 1900’s. Googling brought up the census numbers. The 1990 US Census estimates 19.8 million foreign born citizens in a population of 248.7 million, 8.0% of the US population. At the peak of the last wave of immigration, there were 13.5 million foreign born citizens in a US pop of 92.2 million, making up 14.6% the country. And this wasn’t an isolated peak, the foreign-born percentage was 9.5% in 1850, 13.1% in 1860, 13.4% in 1880 picking the earliest figures I found and then the other high points (pdf, pdf).
So the rate of immigration into the US is not near the historical high. And in context and with regard to assimilation I would expect current immigrants are easier to assimilate. Typical immigrants today come to the US knowing quite a bit about the country, and already familar with many aspects of the culture from the movies, TV, and books that the US exports around the world. The immigrants coming from Southern and Eastern Europe in the early 1900’s knew much less about the US.