On my ATI card, there is a slider for Flicker Removal
called LUMA. At the minimum setting, you do notice the flicker a lot. Near maximum, you can see how much more steady the picture gets... yet, the details get a little blurred.
I think it may have to do with the rate at which the signal is ran from the card to the tv. In the ati cards, I think they may institute a method of interlacing (splitting the picture into two parts, and displaying each part every other frame)... to achieve higher detail levels... and of course, that makes the picture much more flickery.
I havnt noticed any strain from watching the tv whle playing tho. Tho I do sit at least 4 ft away form the 27" display. I think the only time I notice a strain, is when I was trying to read something in a resolution of 1024*768 - genral windows... (thats probably when the interlacing goes on)