I blogged last year about Robert 'Bowling Alone' Putnam's research which suggested that the level of trust in a community is inversely related to the level of ethnic diversity.
He presented a paper with some of the data here.
(Professor Putnam is still optimistic though. As far as I can see the reasoning goes - America is a successful society. America has had lots of immigration. Therefore lots of immigration is the recipe for a successful society. It's not recorded if the Native Americans were asked for input.)
(name corrected - thanks)