Over at the Originalism Blog, Mike Ramsey discusses whether the Obamacare tax is a direct tax and therefore unconstitutional because it must be apportioned.  Mike links to some work by Jose Alicea.   Another discussion of whether the tax on not having health insurance is a direct tax is this one by Rob Natelson.  Here is an excerpt:

My book, The Original Constitution: What It Actually Said and Meant (2d ed., 2011), pp. 159-61, contains what may be the most complete compendium of Founding-Era sources on the distinction between direct and indirect taxes. While there were some exceptions (for example, although taxes on ownership of capital and household goods were direct, excises on ownership of luxury goods were indirect) the usual line of distinction was that direct taxes were imposed on status, while indirect taxes were imposed on transactions. A tax that one must pay despite doing nothing is the quintessential direct tax.