It won't happen though, you think any ISP would block/not offer HD streaming to the biggest video sharing site in the world and survive? Customers would leave in the millions.
As you say they are businesses they wouldn't do it, it would be suicide, that's why I think it's stupid to believe it would happen.
Since I referenced a product that doesn't even exist (BTTube) it should have been obvious that I was referring to a future marketplace, where ISPs are acting much more as content providers. Look at BT and TT if you want to see that happening today.
As content providers - or as Platypus says as partners to "preferred content providers" - they may well use their power as an ISP to advantage their own services/their partners services over competing services.
Anyway, perhaps YouTube is a very bad example due to the difficulty even Google have had monetising that particular service.
Something like Spotify, or Amazon Prime, would be a better example. If BT decided to become a competitor in music, TV or movie streaming, they could well decide to advantage their own service with better bandwidth. In fact why would that not be extremely likely?