I'm at uni in New Mexico (well, for another few days at least) and the residency requirements are 1 year of residence in the state for out-of-state US students. At the moment, I haven't been able to find out whether international students can qualify for in-state tuition after 1 year or not - I'll keep looking though.
As an international PhD student, I am actually considered to be a resident for tuition purposes because the department pays my tuition as part of my funding and can't afford to pay international fees (however, I am not considered a NM resident for any other purpose, such as scholarship applications etc.).
Edited to add: I've just found a page on the UC Berkeley website to say that international students cannot gain residency for tuition purposes and have to pay full tuition for the whole period of study - not sure if this is the same in every state though (am getting a bit confused by the info on NY university websites).