I've been hearing a lot about the LCD HDTVs with 120Hz refresh rates lately, but have yet to seen one. Among others, JVC, Sony, Panasonic, Hitachi, and LG have some models that have 120Hz. Will it make that big a difference on motion? IF it can be had for just $100 more (everything else being equal), would that be worth the upgrade?

I see a lot of praise about them:
http://tv.about.com/od/lcdproductrev...C_LT37X987.htm
http://gizmodo.com/gadgets/home-ente...ter-231872.php
http://gizmodo.com/gadgets/home-ente...ber-233642.php

Anyone here have any experience/knowledge about them?

The good thing about them is that they should be able to handle 24, 30, or 60 fps without judder, since 120 can be divided evenly into those numbers.

Other than some computer games (especially simpler games or games running on overkill hardware), nothing actually outputs at 120hz. I don't think repeating the frames would make a noticeable difference on a non-CRT, but then again I haven't seen the sets.

Exactly.

Motion issues typically came from LCDs slower than the 60Hz TV signals, and should not be a problem with anything 60Hz (16ms refresh) or faster.

The even multiple is key to get rid of some potential scan-line issues. Although the interlaced channels will still have those regardless of your TV.