All flat-panel TVs are HDTVs
HDTV is not just another pretty flat-panel. As described in the TV Buying Guide, flat-panel TVs have slim-profile screens, and most are either plasma TVs or LCD TVs. While each of these flat-panel technologies can provide higher-resolution images than CRTs, that does not mean all flat panels are HDTVs.
The first requirement for an HDTV is a set that has the appropriate screen dimensions -- high-definition sets in the United States use a 16:9 ratio.
The second requirement for an HDTV is the ability to accept high-definition input, and to be able to display that input at one of the native HDTV resolutions, as described in How HDTV Works. This may mean an HDTV-ready display that requires connection to a digital receiver or antenna, or a set with an integrated HDTV tuner with a built-in digital antenna for over-the-air DTV (which may or may not include some HDTV broadcasts).
Read the information on any flat-panel TV you're considering to see whether it's HDTV-ready or if it has an integrated HDTV tuner built in.