The RAND Corporation has a new report about illegal immigration and whether it's a good idea to have local police doing enforcement. A copy of the study costs about $7, though there's a decent summary here. The big problem is there are so many illegal immigrants and not enough "bandwidth" in the federal justice system. ICE estimates it would cost $5 billion and 15 years to catch up. (So, when you see a protest sign saying "They Can't Deport Us All," that's a fairly accurate assessment.)
RAND doesn't dismiss the idea of having local police do immigration enforcement. But there's a danger for local jurisdictions, beyond the civil-rights concerns.
A 2007 partnership between Immigrations and Customs Enforcement and the sheriff's department in Maricopa County, Arizona, resulted in deputies identifying 16,000 illegal immigrants among 106,000 jail inmates over three months.
However, the effort racked up a debt of $1.3 million in only three months, the percentage of crimes the department solved dropped and the time it took for officers to respond to calls for service grew.