Arizona has just passed a law which creates a 'reasonable suspicion' standard that allows law enforcement to stop those they belive may be undocumented immigrants. That means law enforcement can stop anyone who, on sight, is possibly an irregular migrant.
It is now a misdemeanour to lack proper immigration documentation. So even if you are legal, not having proof of that could land you in hot water. Making this a misdemeanour is what allows the 'reasonable suspicion', as otherwise, the 'suspect' would have to be implicated in another crime before law enforcement could demand immigration papers.
It's a beautiful example of cirular legislation. Can't stop latinos on the supicion that they are illegal if they're not doing anything wrong, so let's make not having papers about your person illegal!
An addition to the bill makes it impossible to ignore the law, and empowers citizens to ask for an order that the law be enforced.
Don't worry though, if you're an irregular migrant with white skin, you'll probably be okay.