As soon as President Barack Obama signed his health care reform bill into law, attorneys general in some states were expected to challenge its constitutionality, primarily on the grounds that the federal government doesn't have the power to force people to buy something -- in this case, health insurance.
But the "mandate" isn't really a mandate. No one is going to kick you out of the country if you refuse to buy health insurance. What they will do is charge you higher income taxes if you don't have health insurance.
Conservative opponents of health care reform may claim that the government has never before forced Americans to buy a particular good or service, but it has, many times, charged people more or less in income taxes depending on whether they do. One could just as easily argue that the federal government requires people to take out mortgages to buy their homes, or to pay student loan interest, or to purchase energy efficient appliances. All of those things have the same net effect under current law that buying health insurance will under the reform bill.