YJ,
From what I have read and heard and seen during press interviews, the AZ law goes a little further than the Federal law in practical terms, inasmuch that it allows profiling of non-Americans, or those that appear to be such.
For example, as you mention, if an American arrested they are asked for ID, and usually some form of State ID would suffice, they would not, for example, be required to produce a green card or passport. Now consider an apparent non-American being arrested, ok lets take the bull by the horns and say a person of apparent Mexican descent is arrested, and asked for ID. Do you believe they will simply stop at the usual State ID? I'm not too certain.
In reality, people like Mr F and myself are just as affected by this law as anyone from South of the border, and more so than the ordinary Joe-public American, as we are by many other laws and regulations, but we accept they are as they are. For example, being in the IT industry, I know that I cannot gain work for most companies that deal with US Military contracts, as I am required to be a Citizen for that, although I currently do work for such a company solely due to my being employed by them prior to 9/11, and so not having to 'prove' citizenship to gain such employment.
I agree that the Az law was brought in due to the lack of Federal resources to deal with the immigration problem, though I would prefer to see something more substantial on the Federal level myself.