100Hz scan is not HDTV, that's for certain. (still having bad memories of that Samsung Tanto, eh stringfellow?). An interlaced picture consists of an odd and even field. A 100Hz Scan TV (for eg, Panasonic Tau Giga) takes each field, and fills in the gaps by repeating each available line. This is called line-doubling. A line-doubled field becomes a "frame" as in "frames per second". The newly reconstructed frames are then displayed twice, thus achieving 100Hz or 100fps. On certain TVs, when playing videogames there may be a noticeable lag between controller input and on-screen action.

Obviously, line-doubling makes vertical "jaggies" even more apparent. Sometimes line-doubling is called "bob" de-interlacing. Because the odd and even lines are offset by 1 line, the even "frame" ends up one line lower than the "odd" frame. When showing still pictures, the image may appear to "bob" up and down.

The Society of Motion Picture and Television Engineers (SMPTE) HDTV standard fixes the vertical refresh rate at 30Hz for all the HDTV modes (480p, 720p and 1080i). 30Hz may sound a bit low and flickery, but new screen technologies like LCDs and Plasmas and upcoming stuff like OLEDs and CNT-FED aren't subjected to flicker like CRTs are. CRT based HDTVs also use frame doubling to up the refresh rates to a more respectable rate, but since they are working from actual progressive image sources, they don't suffer for it like the "100Hz" sets.

edit: 480p is EDTV, oops