I've never heard of a TV with a native resolution of 720p. Its the incoming signal that has a native resolution. This TV will display a signal in 480i (Standard Def), 480p (DVD), 720P (HDTV) or 1080i (HDTV), depending on the source. SD at 480i on this TV looks considerabley worse than a HDTV signal, becuase you are actually viewing 480i, not an upscaled image. I have an upconverting DVD player, and I would say the signal is very marginally better when upscaled to 720P or 1080i from the native DVD resolution of 480P signal that has not been upscaled. The difference is not huge.
LCDs may have a native resolution of X x Y pixels, but there is no "P" or "i" involved