My inclination is that Arizona will be the flashpoint. It won't happen until 2013, IMO, because then the Left will realize that their dreams of a socialist USA will be on the scrap heap of history--and they'll be pissed.
Or someplace like AZ.
As the socialists lose power with Americans they will seek to retain power by any means including shoving voting rights for illegals through the courts or some other extra-legal process.
States, especially those with heavy illegal populations will push back. The illegals and their enablers--the ones we saw on May Day and Cinco de Mayonaisse--will riot...again. When the states seek to quell the riots Obama will move federal troops against the states' using such "heavy-handed tactics."
Then a massive epidemic of "Screw You Flu" breaks out amongst federal troops as they report to sick call too ill to carry out their orders.
From there Obama is impeached or resigns and suddenly America is saddled with it's single greatest constitutional crisis since 1860.
And the Amadinejads and Chavezs of the world rejoice as America spends the next decade rebuiling its political insitutions.
Anyway, if I were to write a story about it that's kinda how it would go.