Mark Cuban talked about the connectivity issues of getting TV from the PC over to the TV. Dave Winer weighed in, mostly in agreement. I'd also agree that it's hard, and unlikely to spread quickly - but for another reason, which Scoble touched on, but didn't really explore.
It's about the complexity. It's hard to connect the current range of dedicated devices to the TV and get things working the way you want. 15 years ago, anyone could walk into any room in anyone's house and figure out how to work the TV. Now? It's often different across different rooms in the same house (use this remote, no not that one - make sure the correct input is chosen, no not that one, you can't change the channel while the DVR is operating...)
It's complexity squared now. Add in a PC, with all the attendant issues? Most people don't want to babysit a PC while they watch TV. We have a Media Center PC in the living room, and getting that to work with the TV was an exciting task - gosh knows how much worse trying to deal with HD would have been. Even now, the PC sometimes can't pump sound to the TV (oddly, sound for other things will still work), and it'll need to be rebooted - or everything it records will be silent. That's a load of fun when you only realize the problem after a show has started recording.
I have no idea whether an Apple offering in this space will be better (I suspect it will be), but the "which $%^&* input do I use now??" problem won't get any easier. What people will likely go for is a dedicated box, which is why I said Scoble touched on it with his points about the XBox.