I wrote a guest editorial on this subject, http://www.theatlanticright.com/2010/03/26/the-constitution-and-taxes/
Republicans are focusing on the 10th amendment problems because they want to make a more general argument about the Constitution since they think its been abused regularly by Congress and the Court. But its actually the more difficult argument to make -- the easier argument with it boils down to the fact that our Constitution limits the way in which the federal government can enact taxes.
Congress can't just pass any tax they want, thats why the Sixteenth Amendment needed to be ratified to allow the income tax. A fine for failure to buy healthcare be prohibited for the same reason that an income tax was at that time.
I explain why at the link.