TV programmes debuted on the UK iTunes store today. At the moment you can buy (for 1.89) programming from ABC, Disney and MTV amongst others. More info here.
Until we see some BBC and Channel 4 programmes on there, I don’t think this will really go mainstream, although some big shows like Ugly Betty and Grey’s Anatomy are on there now. The question is will that ever happen? Broadcasters here are already working on their own commercial venture for programming which is more than seven days past its transmission date.