The Wall Street Journal's editorial board, in an excellent piece, points out the high stakes involved in the constitutional challenges to Obamacare:

All human activity arguably has some economic footprint. So if Congress can force Americans to buy a product, the question is what remains of the government of limited and enumerated powers, as provided in Article I. The only remaining restraint on federal power would be the Bill of Rights, though the Founders considered those 10 amendments to be an affirmation of the rights inherent in the rest of the Constitution, not the only restraint on government. If the insurance mandate stands, then why can't Congress insist that Americans buy GM cars, or that obese Americans eat their vegetables or pay a fat tax penalty?

The mandate did not pose the same constitutional problems when Mitt Romney succeeded in passing one in Massachusetts, because state governments have police powers and often wider plenary authority under their constitutions than does the federal government. Florida's constitution also has a privacy clause that underscores the strong state interest in opposing Congress's health-care intrusion.

As for the assertion that the mandate is really a tax, this is an attempt at legal finesse. The mandate is the legal requirement to buy a certain product, while the tax is the means of enforcement. This is not a true income or even excise tax. Congress cannot, merely by invoking a tax, blow up the Framers' attempt to restrain government under Article I.