It sounds like you're talking about Windows Media Centre running on the PC integrating with Mediaroom on the set top box. Although Microsoft says this is possible, I'm not aware of any provider actually doing it, certainly no one in Canada has that feature.
Thanks for your responses everyone. I'm guessing there is no point in putting a TV tuner card in the HTPC I'm building. Shame because it would've been nice to get rid of one of the boxes in my entertainment center. I might even just hold off and keep using the PS3 as my blu-ray, WDLIVE as my media player, and the STB for TV.