If you see a TV that advertises 1080i/720p I would only guarantee that it accepts those resolutions and scales them appropriately to it's native resolution. Only trust the "native" resolution of the panel.

As Mark says there is essentially no such thing as a "720p" panel. About the only 720p display I've seen is a projector.

Apart from the Hitachi/Fujitsu panels you can't get a 1080i flat planel. Everything other panel is always progressive and the internal processing will need to de-interlace and scale (in the case of 768p) a 1080i signal and scale a 720p signal (in the case of a "768p" or "1080p" panel).