Odd, while it makes sense I would have assumed that in this day and age almost every TV broadcast would be in stereo at the very least. Yet almost everything ends up being collapsed to center. I can't ever really detect a time where it takes a standard two channel input and actually matrixes it out into 4 or 5 channels. Given the rare occasions where it actually works I've always just assumed that these were likely encoded for Pro Logic. The really odd thing has been that when watching newer shows (i.e. the type that specifically mention how they're shown with surround sound and other such things that aren't supported in my system due to my cable setup) they're also collapsed to mono. I'd say it's a pretty strong bet that they'd be in stereo at the very least.

Something I have thought of is that since it was coming through TiVo the compression process might have stripped things down enough that it affects the sound, but double-checking with OTA has proven that this isn't the case.