Actually nVIDIA has a reference design using a different Connexant TV-Out chip, which would output Y/Pr/Pb using three RCA jacks. It's just that no graphics card companies have felt the need to make a card with it yet. Probably not much of a market just yet.
I'm looking forward to finding out about their new Personal Cinema stuff. Hopefully they will try to compete with ATI's All-In-Wonder stuff with DirectX 9. nVIDIA's been teasing us with the idea of a cheap HDTV tuner card for years, but it's never materialized. I imagine they were waiting for all of the High-Def Copy Protection and digital rights management stuff to get into place. Look at how MP3's exploded and all the litigation that's ensued. I think aside from the market being small, they wanted to protect themselves. Maybe next year we'll finally see some HDTV noise out of nVIDIA.
For now ATI is the only way to get PC-to-HDTV output unless you're fortunate to have a TV that takes VGA input or the five connector RGBHV BNC jacks.