No, Congress can't just decree that individuals must buy a private product, even if the market has unique properties. To do so would be more than unprecedented; it would be an unconstitutional overreach. That's the gist of what an 11th Circuit appeals court said today when it ruled in favor of 26 state governments by saying that the federal requirement to purchase health insurance contained in last year's health care overhaul is unconstitutional. From the ruling:

The individual mandate exceeds Congress's enumerated commerce power and is unconstitutional. This economic mandate represents a wholly novel and potentially unbounded assertion of congressional authority: the ability to compel Americans to purchase an expensive health insurance product they have elected not to buy, and to make them re-purchase that insurance product every month for their entire lives. We have not found any generally applicable, judicially enforceable limiting principle that would permit us to uphold the mandate without obliterating the boundaries inherent in the system of enumerated congressional powers. "Uniqueness" is not a constitutional principle in any antecedent Supreme Court decision.

However, the panel, made up of two Democratic appointees and one GOP-appointed judge, did overturn lower court Judge Roger Vinson's decision to invalidate the entire law, preferring to strike only the mandate and related provisions.

More on previous decisions for and against the mandate here and here.