On June 25th, 2012, the United States Supreme Court decided upon the constitutionality of the anti-immigration laws enacted first by the state of Arizona and followed by other states, in the decision Arizona v. United States. The court has struck down important provisions of the infamous law and has basically concluded that immigration law is (still) a federal law and thus any state law that interfere with the federal provisions in this area of law is unconstitutional. And since the immigration law is a federal law, it must be enforced by federal agencies. Thus, it is unconstitutional and illegal for the local police to racially profiling people and make stops based on their suspicion that the person might be "illegal". Unless the police stops every single car and verify the legal status of every single person, they would not be able to prove the legality of these stops.
This decision represent a great defeat of the anti-immigration state laws that have been enacted in the last year by several other states. Georgia has also enacted a similar law with Arizona's in 2011.