Working on a construction crew back in college with a few workers each from Mexico and Guatemala, I was amazed at the animosity between the two groups. They would joke, not good-naturedly, about how much cheaper the prostitutes were in the neighboring country or how stupid the other's politicians were. I traveled in Central America a few years later and found the same thing.
This great article from earlier in this week's LA Times shows the economic and cultural effects of these nationalist tensions in the U.S. It chronicles how immigrants from El Salvador have to assimilate to the existing Mexican power structure in Los Angeles for jobs. Since the dominant Latin culture in LA is Mexican, and there is a strong nationalistic bias in some communities, El Salvadorians are changing their accents and even adopting their cuisine and mores to fit in.
The reverse is true in other cities. When one of the Mexican construction workers I knew flew out to D.C., he entered a taqueria in the Adams-Morgan area. Upon hearing his accent, the cashier from another Central American country promptly said, "The Mexicans eat over there." Looks like you better choose your adopted city carefully if you're a Central American immigrant.