Hey,
I have the exact same problem... i own an intel imac 20", and since the moment i opened i saw those anoying horizontal lines that appears when you watch a video in fullscreen, in the flurry screen saver, wathing TDT with eyetv, playing Doom 3, etc.
It´s like a desincronization of the images, also i´ve nocticed that it happens when the image is moving fast.
I called apple a couple of weeks ago, but because i´m at work during the support time-frame, i wasn´t able to do tests. I tried to describe the problem to them but they didn´t know what i was meaning with the "glitches" or "tiny horizontal" lines.
At the end, they suggestesd to reset the PRAM (which didn´t do anything at all), and also, trying to reboot from the disk (what I don´t understand how am i gonna test video from the instalation disk)... and if nothing works, take it to certified tech in Spain, but first, be sure it´s a hardware problem.
I´ve red somewhere else, in macnn forums (i think) something related to this, inside a thread talking about the problem with 256 and Front Row, someone was explaining the same problem... but now i cannot find it.
So if anybody else have the same problem or know anything helpful about it... pleaaaaase, help!!!