I think this law is sorta like the tax on pot. It is just one more way the government can prosecute. In this case an illegal alien would be prosecuted for both a federal and state crime.
One can not be tried for the same crime twice, but in making this a state law, it gives the prosecutors another charge in addition to the federal charges.
This also clears up the gray area of whether state officials can prosecute federal laws when the federal prosecutors choose not to. This is especially critical when considering how state law enforcement can enforce 8 U.S.C Section 1101 which is entitled The Immigration and Nationality Act (INA).
The CRS published a very good report on the complexities of state law enforcement enforcing the INA.
Enforcing Immigration Law: The Role of State and Local Law Enforcement
http://www.votolatino.org/reports/th...nforcement.pdf
It should also be noted that this Arizona law will probably be challenged in the Supreme Court as Congress is empowered by the Constitution to "To establish a uniform rule of naturalization".
It will be an interesting case to follow.