Same problem here. For me the pixelation began about two months ago but I'm unsure of any correlation other than a change in season. It's not the TV's since I can play PS3 games and movies and the same goes for the Wii on multiple in home TV's. Happens on all my TV's all of which are different models. I have two HD STBs and two that are not. Doesn't appear to be related to the type of material/content being viewed meaning graphic intensive / light to dark extremes.......it seems to be channel specific and affects the more commonly viewed channel. When I tune in to obscure cable programs I don't have any issues.
I have not called for service yet as I'm dreading the run around that I now they'll give me. I work all day so I can pay for services like Fios and I know that my call will result in premise equipment troubleshooting like STB reboots, cable modem resets, cabling and splitter change outs etc...........I've read a number of other posts from folks having the same issues which occurred spontaneously after years of quality service. I dread the run around I'm going to get.
FIOS! FIX THE PROBLEM