Well, 480p is a 4:3 picture, and is technically part of the HD specification. I don't know if anyone does it, oreven if it's possible for a single channel to switch back and forth, but it's vaguely possible that Tom's local Fox station does.

I imagine you're right, though, and that they pillar box the 4:3 picture into a 16:9 frame.

Personally, I'm a little disappointed with how my HDDirecTiVo deals with this stuff. I wish it would automatically produce the same output as input, or at least show me what the input was. For now, I have to guess what the original format was based on letterboxing, etc.