No not necessarily, all signal goes to Digital in 2009, which is not HD. You can buy TV's that pick up DTV but are not HD, my daughter has a ETV (Enhanced TV) that are a lot cheaper.
It's quite a bit more complicated than that. All terrestrial signals must go to ATSC by Feb 19, 2009. ATSC defines 18 different formats (resolutions). The lowest is 480i at either 4:3 or 16:9, which is roughly what you get now. The highest is 1080p. A station can broadcast any of those 18 resolutions, or any combination of them on different "sub" channels. EDTV is basically 480p, which is what most DVDs are encoded at, and is one of the formats defined by the ATSC standard.
An ATSC tuner should receive any of those formats, and, optimally, transcode it for output to whatever device you have. If you have an SDTV, it will downconvert it and output it to that TV.
The VAST majority of terrestrial stations will be broadcasting in HDTV (720p, 1080i or MAYBE 1080p) come 2009, since there is no point in not doing it.