Good to hear you say that, assuming you didn't also buy an upconverting DVD player. In my experience, the HD TV does as good a job of upconverting as a DVD player. Your mileage may vary, of course.
The DVD player is putting out 480i, and the TV (a Toshiba “Regza”) does the magic. “Full Screen” DVDs look a little odd, with big black space on both sides of the picture. “Wide screen” DVDs almost fill the screen, there are very narrow black spaces above and below the picture. In neither case is the picture distorted, and it looks much better than it did on my old tube TV.