I don't have time to fully dissect Arizona's new law requiring police to question people if there's reason to suspect they're in the United States illegally.
But I will toss out this relevant legal factor that I haven't seen mentioned in the discussion: any arrest made under the Arizona law needs to comply with the Vienna Convention. Specifically, if a foreign national is arrested, prudent law enforcement officials will read the arrestee his or her "Vienna Convention Warning," which essentially informs the person of a right to contact consular officials.
I think the chief hurdles the law will face in court will stem from its perceived overbreadth (is reasonable suspicion enough of a bulwark against potential civil liberties violations? or is it too much discretion?) and from the federalist notion that the federal government generally supersedes when it comes to border and immigration issues.
And then there's a provision like this (*Added: see the explication of my thoughts on the following provision in the comments):
E. A LAW ENFORCEMENT OFFICER, WITHOUT A WARRANT, MAY ARREST A PERSON IF THE OFFICER HAS PROBABLE CAUSE TO BELIEVE THAT THE PERSON HAS COMMITTED ANY PUBLIC OFFENSE THAT MAKES THE PERSON REMOVABLE FROM THE UNITED STATES.
I don't think the law is quite as draconian as some of its more outspoken critics make it out to be, but it does have some aspects that may be asking for trouble should they come before a court. Still, as usual, I think a large portion of detractors have immediately painted the law as motivated wholly by racial animus, which clouds discussion and debate. Regardless of race, if illegal aliens/undocumented persons are, for example, committing crime at a rate the state finds unacceptable and the federal government isn't enforcing the border adequately, I think the state has a rational basis for passing the law.
Speaking very broadly, I think any push toward national immigration reform (which seems to be the intended Democratic response to the Arizona bill) should reduce the hurdles - especially the lengthy timeline - to legal immigration to the United States. But in doing so, we should also demand much more rigid adherence to the law (i.e., penalize and enforce provisions against illegal immigration all the more) once it's actually easier to enter the country in a legal manner.