I think this is largely being overblown. It's already a federal crime to enter into this country illegally; AZ did nothing to alter that. People are worried about an ad hoc legal approach to enforcing these laws, but...there's already a ton of discretion with law enforcement (and the state and federal government, for that matter) in enforcing the law. If there wasn't, this wouldn't be an issue in the first place. But since the federal government mostly refuses to enforce the laws on the books, AZ has taken the law into their own, uh, hands.
I'm just waiting on the grand idea to fix this whole illegal immigration problem from the Left. I'm sure it's coming...some day.
Don't hold your breath, the left sees it as a huge voting block.