Microsoft announced today
that they have partnered with Time Warner to bring their TV app to the Xbox 360. The app will allow you to watch up to 300 channels on your Xbox 360 without using a cable box. I'm guessing this will be leveraging their existing iOS and Android applications which allows you to watch most of their TV channels (as well as shows stored on your DVR) from a device anywhere within your home network (i.e. you won't be able to take your Xbox 360 to a friend who doesn't have Time Warner and watch shows there).
You won't be able to watch all of your channels on your Xbox 360 but you will get a good selection of channels like AMC, BBC World News, Bravo, Cartoon Network, and Comedy Central. So it's not a complete replacement for a cable box but it does add a little bit more value to using your Xbox 360 as a media extender.