100hz is basically a form of frame prediction/interpolation combined with faster screen refresh (and therefore screen response) when 100hz mode is on, then the tv takes the standard 50hz signal, delays displaying it by at least 1 full frame for progressive inputs (or 2 fields for interlaced). this is so that it can look at the next frame and the last frame side by side, and interpolate an in between frame. it then displays the first real frame followed by the in between frame (which it made up) then the next real frame. whch it does at 100 times a second which is effectively doubling the frame rate.
In some systems, there may be other processing applied to the interpolated frame as well (such as exagerating some features) in order to make the transitioning of the liquid crystals less noticeable which reduces blur.
Also, TVs equiped with this also need to have better panels which can be addressed at the higher speed necessary (i.e. shorter response time) which also reduces the blur of "standard" LCDs.
All of this seems to work quite well on the LCDs that I have seen it on, and makes motion much easier on the eye (I find especially in movies when they do the "running with a camera in a war zone while bombs are going off next to it" camera technique)
The only problem is that this introduces an extra processing delay (while the tv waits for the next frame) which is not a problem for tv or dvd etc, but can appear as a small time lag when viewing something fast and interactive (eg a computer game) but the time delay is still fairly small and you may not notice it at all. In fact some LCD/plasma TVs already have a lag anyway due to other processing that they do which may be much greater than the lag caused by the 100hz.
Also sometimes the tv can get the "guessed" frame slightly wrong in places which can result in a few aftifacts becoming noticeable, however I have never seen any of these.
Its best not to look too closesly for the artifacts/lag since you may not ever notice them, but if you look hard and find them, chances are you will always notice them from then on (which is a real pain since it means you will need to get an even better telly for them to go away!) Ignorance is bliss when it comes to audio and video artifacts.
I have found the 100hz feature on my panasonic 26lxd70 to work very well (I upgraded from the lmd70 due to viewing angle and motion issues) and there doesn't seem to be enough lag time for me to notice. If you are really worried, then go for a TV that lets you turn off the 100hz mode. The panasonic doesn't let you turn it off, but it has bee flawless for me so far anyway.
If absolute minimum lag is a must for you playing games, I hear that samsung tvs have a game mode which cuts out most of the processing for faster response, but since I havn't used it I don't know how well it works.
Hope that helps
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