Here.
________UPDATE________
Aside from Michele Bachmann grandstanding stupidities, there also were Fifth Amendment concerns in compelling persons in a civil law matter, the census, to admit to crime, illegal entry into the country.
Knee-jerk GOP sorts would tell you illegal entrants are the equivalent of non-uniformed combatant detainees, having no Constitutional protections because they are not US of A citizens; but marginalizing basic rights (thought necessary at the start of our current federal alignment) is simply bad for ALL people who value freedom from too heavy a government foot on things.