Andy, thank you for your kind words. I defer to you on the latter part of your post regarding unintended consequences, and I agree (and have written, particularly in the Islamist context) that taking an otherwise suspect classification into account when investigating a particular criminal enterprise that is self-defined in those terms is permissible. (I have argued that this is really not profiling at all).
But I think it is you who are comparing apples and oranges in applying that principle here. The problem of illegal immigration from Mexico is not really that kind of enterprise. It’s troublesome to say the police can identify a Mexican (or Italian, etc.) by appearance, and I’m uneasy letting the government define a problem in narrowly racial terms and then claim that it is entitled to consider race in combating it. This is like a police department saying that it is concerned about black drug dealers (they are the ones selling the drugs in its jurisdiction), and so it will target blacks. But most importantly, all this really goes again to the permissibility of considering race as a matter of law, and doesn’t answer my concerns about the divisiveness of this sort of discrimination as a matter of policy.