Pixellation occurs when the bandwidth cannot keep up with the amount of data it is expected to carry. This is most frequently a signal compression issue rather than a television problem. I first began to notice it on my old ANALOG TV when my cable provider changed over to digital signals.

You may have noticed it most severely in that fashion show because of all the flash photography. Most scenes, even those containing fairly rapid motion, have large areas that don't change as the picture goes from frame to frame. The areas that keep the same color and luminosity are amenable to high compression ratios. When you get a lot of flashbulbs popping, however, you need to rapidly refresh EVERY SINGLE PIXEL on the whole screen, and this requires much more data to be transmitted, more than the system can handle. So they deal with this by making the "pixels" bigger, and thus the pixellation.

You have a Sony XBR4 -- so do I. It is NOT your TV. I don't get any such effect even under extreme conditions on HD content played on either my Blu-Ray or HD-DVD player, which are connected via HDMI cables. I have enough bandwidth there.