The Court found Obama-care constitutional under the power to tax for the general welfare. The clause is found in Article one, section eight, clause one.
The power to tax is not the same as the power to nationalize health insurance.
The sad thing about the Courts decision was that it reaffirmed the dangerous shift in our governance toward decidedely unequal protection. Roberts specifically cited the presumed legitimacy of government power to use discriminatory taxation as a means of coercing behavior - and this is where I believe the core err lies. Roberts saw that ruling against this kind of taxation on equal protection grounds would unravel a huge portion of the federal government's ill-gotten power, because pretty much every other 'tax incentive' is based on the same concept - that the government can arbitrarily dictate behavior and use discriminatory taxation as punishment. In my view, this does an end-run around the fundamental concepts of limited government and equal protection, but I guess, from the point of view of those eager to expand government, that's the point.