I am in the construction industry, so it goes without saying that I know someone that has illegals working for them. Most people do know someone that has illegals working for them. That is really a useless point of information.
I agree the decision to hire someone that one suspects or knows is an illegal is a moral issue; but that does not mean that the obligation should be put on employers to assure the legal status of their employees. If the employee can present sufficient documentation to allow the employer to pay taxes on behalf of the employee (another thing that should not be an employers responsibility), then his/her obligation should be considered complete.
Employers are not and should not be considered obliged to be bookkeepers or enforcement officials for the government.
Any construction firm that needs to hire illegals as part of their business model should just go out of business. If their margins are that tight then it is NOT a well run organization.