Obama was arguing with George Stephanopolous (ABC News) over if the government forcing people to pay for something on penalty of fine is a tax. Obama says it is not, but Stephanopolous does not seem satisfied with his lack of argument. Privet company can not tax, and just because your critics accuse you of a lot does not negate this particular claim. In the end, it seems a semantics difference at best. Under Obama's likely plan, you are going to be forced to pay or be fined. Even if you do not want to call it a tax it has the same effect.
Video embedded below. (from http://www.floppingaces.net/2009/09/20/a-tax-by-any-other-name-obama-stretches-the-truth-about-forcing-people-to-buy-insurance-or-be-fined)