There's actually a lot of emerging technology right now to get IPTV into the mainstream.
If you think about it, YouTube proves that there is a huge demand for video over internet. Enough for Google to make a big play on it anyways. What's lacking now is participation from the major medias.
I know they have their own ideas on it, but frankly, it's anything but centralized like regular television.
What's lacking are also the intuitive, no-fuss software client to handle it. Although it's not the only one, there's a program out there called Joost which is attempting to do just that. You can sign up and watch it for free. I've tried it, and I think they're on the right track. However, both the software (and even the content) are in Beta, and it shows.
Apple's movie trailers also show that HD content can work and be displayed beautifully over the PC.
So, we just need someone to put all these piece together: The right business model, corporate media participation, and technology. No big deal.
I think the best implementation of IPTV I've see so far is from tv.yahoo.com. However, they're not so much as trying to replace TV as they are a supplemental means for corporate media to advertise their shows. Otherwise, there's no reason why it can't work for full episodes of shows (and there may be some back shows that are available, but I haven't looked).
I honestly believe that IPTV is the wave of the future. Yes, IPTV. Not regular HDTV as we know it now, but IPTV. Imagine an integrated solution between the ease of use of regular HDTV with the functionality of DVR and the versatility of shows-on-demand like a PC download all built in one! Or... in and ideal world, it should be....
The only real thing that's holding it back-- but it's biggie-- is participation from Big Media who are afraid of losing control of their content....
Still, if you're feeling adventurous, there are plenty of IPTV stuff to find online. Weird, quirky, and slightly frustrating sometimes, but they're mostly free!