I have a weird problem ... not sure it is actually a problem, but let's see what you all think.

Brand new Sony XBR4. When watching through the Series 3 Tivo (and only when watching through the Tivo), there is a very think sliver of pixelation at the top of the screen, in SD programming. This mostly happens on lower numbered SD channels, but can occasionally be seen in SD programming on HD channels (i.e., a non HD program that I'm watching on the ABC HD channel).

Changing the setting from full pixel to normal or the wide mode from full to any of the other settings but one gets rid of it -- but at the price of a more distorted picture.

Any ideas?

That pixel line is part of the picture signal. Search the XBR4 thread for "overscan" and you will see several dozen posts about this.

That line is usually hidden with overscan. However, if have the XBR4's HDMI input set to "no overscan" mode, you are going to see those artifacts at the top of the screen. Why don't you try the overscan or +1 format setting on the Sony?