Basically if your HDTV doesn't support 24p then your Blu-ray or HD DVD player will have to output to the TV at (usually) 60Hz instead. Most films (on BD/HD DVD) are encoded at 24 fps, so in order to convert that into 60Hz the player has to perform what is called "3:2 pulldown", where it basically repeats the first frame 3 times, the second 2 times, and continues to alternate like that throughout. This results in a mild judder effect which is really only noticeable in slow panning shots (and usually only if you know what to be looking for).
It is worth noting that if you have ever imported DVDs from the US or watched any other films in NTSC then you will have already experienced this - as they have to do exactly the same thing there when they take the original cinema film at 24 fps and convert it to 60Hz. If you didn't notice it before then chances are you won't find a lack of 24p support to be a problem. Here in the UK we never had this before as PAL is 50Hz, so instead of using pulldown, films are instead slightly sped up and then each frame repeated twice to get from 24 fps to 50Hz. Rather than judder we instead have subtly shorter versions of films, with the sound often slightly higher pitch (unless correction was used).
I happen to have the Panasonic PZ70, and while it will happily accept a 24p input - it actually then internally converts it to 60Hz anyway, so you should still get judder. Having said that I've never actually managed to spot it on any of my Blu-ray films and it certainly hasn't tarnished my enjoyment. The PZ70 is an absolutely amazing buy for the price (currently ~£900 online if you shop around). If 24p remains an issue for you then you could wait for the next generation Panasonic plasmas that were announced at CES. The PZ85 and above will all have full 24p support.
Hope that helps.
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