Jonathan Adler at Volokh thinks it may be:

As I understand the current proposals, the individual mandate would operate as follows: A tax would be imposed on all individuals, and the tax would be offset by a credit for those who purchase or are otherwise covered by qualifying plans.  The constitutional problem would arise if this tax is considered a “direct tax.”  Why?  Because Article I, section 9 provides: “No capitation, or other direct, Tax shall be laid, unless in Proportion to the Census or Enumeration herein before directed to be taken.”  “So if mandate is imposed through the tax code, and the provision operates as a “capitation” or “other Direct tax,” it would have to be apportioned.

Do the respective individual mandate provisions constitute direct taxes?  I’m not sure.  “Indirect” or so-called event taxes are not subject to apportionment under Article I, section 9, and income taxes were exempted from the apportionment requirement under the 16th Amendment.  So the question would be whether any tax imposed on those who fail to purchase qualifying health plans would constitute a “direct” tax, or whether they could be properly characterized as indirect or income taxes. From what I understand, the tax in the House bill is, at least for some individuals, based upon income up to a set threshold.  This might be enough to avoid the Article I, section 9 problem

This approach is much smarter than Rivkin’s argument that the Mandates exceeds Congress’s commerce powers. That case is DOA (pun intended). Of course, Pelosi and Reid can simply re-characterize the penalty so it is clearly not a direct tax, and escape the apportionment requirement. But if they fail to do so, totally possible in light of the rapid pace they rahm through this legislation, this could set up a potential facial, or as applied challenge, if enacted.