Andrew Gelman notes that the definition has changed:

It used to be that a millionaire was someone with million dollars. Nowadays, though, the term is often used to refer to people who make a million dollars a year. (See here, for example, in a discussion of the so-called millionaire's tax break.. I guess it makes sense--lots of Americans have a million dollars in assets, and so for "millionaire" to keep its traditional meaning of "really rich," it had to shift somehow. It still seems funny to me, that this change in definition has occurred without it being formally acknowledged.