I think I see now what you're using to define diversity. America certainly does have plenty of immigration, and plenty of "little wherever"s, but really they don't make much of a dent in the Monoculture.
If you're going to argue that Northern Basque communities near Southern Basques communities is diversity, then a "little Korea" next to "little Manila", each with more people, qualifies.
It's not a "monoculture" so much as a dominant one. Yes, one can live within the monoculture, but the ability to make that choice doesn't mean that one could choose to live in a large number of cultures within the US.
And, both monoculture and dominant culture overstates things significantly. Even if one thinks that NYC is one culture, it's not the same as upstate NY's culture, let any of the ones in Texas.
Yes, there are lots of places in the US where one can find very similar cultures, but again, that's a choice. (Ever hear the term "don't Californicate Colorado/Oregon/Arizona"?)
You'd think that folks who claim to value diversity would actually practice it given the choice.
