I just bought a tau today. It does support 780p. What they mean by nothing over 1080i is that it will not support progressive in 1080. So, don't worry, it should be native 780p. If I'm wrong, somebody please feel free to correct me.
Do you have a model number?
Panasonic says none of the Tau support 720p just 1080i for the HD models so make sure you set your Xbox (or Xbox 360) to 480p and 1080i or you will not get a picture.