> I could swear that Polish-Americans are pretty completely
> assimilated, though maybe I've just run into the wrong bunch.
Have you been to Greenpoint lately?
But more seriously, Thomas & Znaniecki are not deterministic - they say outcome depends on economic success, and those more successful assimilate. It is those "less fortunate" :) who fall back on their "old-world" identities and social coping mechanisms. And remember, that the bulk Polish immigration to this country occurred at the beginning of the 20th century - so the folk you meet today are third or fourth generation. The newer immigrants were for the most part dissidents, which is an entirely different story. The Russians, however, who started immigrating en masse to the US just recently tend to live in ethnic ghettoes.
Of course, subsequent generations are a different story since they already have the "host country" identity. They may cultivate their old-world roots, but this is more of the "ancestor cult" than actual social coping strategy on which they can fall back. Of course, there can be exceptions to this too e.g. second generation growing up in an ethnic ghetto with few ties to the mainstream society.
However, the main purpose of my argument is to explain the salience of old national identities in some immigrant groups and the relative absence of it in others, rather than to explain the assimilation success rates (which is a different argument.) The idea is that immigrants from "collectivistic societies" are more likely to espouse such identities than immigrants from non-collectivistic ones, and that is also linked to their economic success in their host country. I think that may work for the Italians as well - the ones retaining their old world identity are probably those from the South that is more "collectivistic," while the more modern northern Italians tend to be more assimilated. Perhaps someone more familiar with that immigrant group can shed more light on it.
Wojtek