Okay, so Billery let the cat out and Obama is about to sue AZ to block the AZ immigration law. About the only angle with potential success for the fed is that immigration law already exists, the fed is responsible for enforcing it and AZ has tread on it. Fed law trumps state law EXCEPT when state law maintains an aspect that is not addressed in the federal version.
AZ's argument, states right to self govern. This should an interesting battle over constitutionality.