Jim Simon's advice is only relevant for Windows users. hey Aaron is clearly using a Mac.
I export to QT often, and I use previews.
The correct answer is "Sometimes it uses the previews, others it just re-renders everything on export." It depends on what switches you throw in the Export window, and what your Sequence Settings for Previews is. If your Sequence Previews are the same codec as your export codec, then your exports should use your Previews, and go faster. It works more reliably on CS6 than previous versions, in my experience.