PlasticBag.org publishes a paper on "Social Software for Set-Top Boxes". Quote: "Imagine a buddy-list on your television that you could bring onto your screen with the merest tap of a 'friends' key on your remote control. [...] The interface that would let you add and remove friends, and see what your friends are watching in real-time - whether they be watching live television or something stored on their PVRs. Adding friends would be simple - you could enter letters on screen using your remote, or browse your existing friends' contact lists."
Coincidentally, Business Week runs an article on Social TV being developed at PARC: "Being developed by scientists at the famed Palo Alto Research Center (PARC), credited with inventing the Ethernet and the laser printer, Social TV will allow geographically dispersed friends to chat and watch TV together."
And comments on Corante point at a similar ongoing research at Alcatel that is building AmigoTV and has published a paper (pdf)