Most US born native speakers of Spanish come from one of two sub-cultures - Puerto Rican or Tex-Mex. Both dialects have been strongly influenced by the US mainstream culture. In colloquial Puerto Rican Spanish, for example, the formal "you" (usted) has almost entirely disappeared and been replaced by the informal "tu" form.

An anecdote to illustrate the signifcance: Some years ago I was leading a SOUTHCOM civic action site survey in Honduras and one member of my team was a female CPT from Puerto Rico. The Honduran military who were supposed to join us on the survey had failed to show and I had recruited a Honduran LT to provide a Honduran face to the team. The CPT began to interrogate him as if he was a raw recruit constantly using the "tu" in one of the most liguistically formal cultures in Latin America. So I intervened and got her out of the discussion and later chewed her out (in private) for using the "tu" with him (which was clearly insulting). She, of course, had no clue that she had done anything wrong and I am not sure that even today she would get it.

Moral of the story: Relying on "native speakers" without any training in the sub-cultural implications of their language is dangerous at best. Or, as Churchill put it when speaking of the Brits and Americans, "One people divided by a common language."

Cheers

JohnT