Here's a New York Times op-ed defense of the law's constitutionality by an author of the legislation, Prof. Kris Kobach of the University of Missouri-Kansas City School of Law. His conclusion: "the Arizona law hardly creates a police state. It takes a measured, reasonable step to give Arizona police officers another tool when they come into contact with illegal aliens during their normal law enforcement duties. And it’s very necessary: Arizona is the ground zero of illegal immigration. Phoenix is the hub of human smuggling and the kidnapping capital of America, with more than 240 incidents reported in 2008."
And here's commentary on the op-ed piece by Jonathan Adler in Volokh Conspiracy. Adler notes:
Kobach’s op-ed does not discuss all of the law’s provisions. One he omits is a little gem, discussed here, that authorizes private citizen suits to force greater enforcement by Arizona government agencies. Specifically, the provision allows for suits against any government agency that “adopts or implements a policy or practice that limits or restricts the enforcement of federal immigration laws to less than the full extent permitted by federal law.” The use of citizen suits to force more stringent enforcement is common in some areas of the law, such as environmental protection, but that’s hardly a ringing endorsement. In practice, citizen suits often prevent executive agencies from allocating inherently limited resources in line with agency priorities. This is bad enough in an administrative context, but should be deeply worrisome in the context of standard law enforcement. This provision is also different from traditional citizen suit provisions in that it provides for monetary penalties.