A great post from Bill Easterly.
This is something I wonder about. The first big wave of immigrants to the U.S. were from the British Isles and didn't need to learn English. Many of the Irish who came in the next wave also spoke English, but the Germans didn't. It's obvious today though that their descendants learned English just fine. But how long did it take? How many generations?
My intuition is that concerns that Mexicans (and other Hispanic) immigrants today will never assimilate are overblown. But there are a two things that have changed that make me worry. It's possible that the pattern of Hispanic immigration in more concentrated than German migration. The more Spanish-speakers that cluster in one area the less there is an incentive to learn English and the greater the probability that such areas will turn into de facto two-language states. Similarly, the government is (as far as I know) much more accommodating of non-English speakers due to multiculturalism: state tests are in multiple languages,, translators are mandated in hospitals, and there is a general emphasis on teaching English-speakers Spanish in the hope that it will make life easier for native Spanish-speakers.