One problem with using police action to control illegal immigrants (rather than effective entry controls) is the likely harassment of U.S. citizens and legal immigrants who happen to appear ethnic. CNN had a short spot today on the expereince of some Hispanic gentlemen, native-born U.S. citizen in one case, who were arrested and detained despite telling authorities they were citizens.
Naturally there's no easy answer to this problem. Rather obviously, the authorities couldn't simply take people's word for it that they are citizens. Anybody could say that, and everybody would say that.
But, the alternative means that U.S. citizens who happen to appear ethnic are subject to constant harassment from immigration auathorities and the risk of being swept up into the legal system at any time. This also seems unacceptable to me.
The bottom line is that trying to enforce immigration law inside the country, rather than at the border, is not going to work without enormous cost.