One American citizen gets kidnapped by accident by a Mexican drug cartel and Arizona passes a law to crack down on anyone that looks like an illegal immigrant? 
No, you said something about crimes committed by illegal immigrants doubling. Got any proof?
That's not the only example of an American citizen being kidnapped due to mistaken identity. I'm curious, how many makes it a problem, though? Are you seriously saying that it's OK for a certain number of American citizens to be kidnapped but it's wrong to crack down on illegal immigrants who are also criminals and the majority of the people committing these kidnappings? Do you not see how stupid that looks?
Last year, 2009, Arizona apprehended or arrested 250,000 illegals who had or were in the process of committing a criminal act other than being an illegal alien. A total of 250,000 arrests in one year in a State of 6.5 million. That's not a problem.

Just under 40% of the population in Arizona is Hispanic. 75% of Arizona residents support this law.
Also to note, about 50% of all police officers in Arizona are Hispanic.