Consumer Electronics Control (CEC) protocol.

CEC is a 1-wire bus running on the HDMI interface providing automatic control in HDMI-interfaced systems.

It's based on AV.link known as the CENELEC EN 50157-1 standard also known under the trade names nexTViewLink, SmartLink, Q-Link, EasyLink.  AV.link was used on the old SCART (EIA Multiport) connectors.


CEC will connect up to 10 Audio Visual [AV] devices. 

PCWorld: The Secret Feature on Your HDTV: HDMI CEC
 From Hackaday:

The CEC protocol is a 1-wire serial bus built into the HDMI standard. The solution he settled up required one solder connection on the motherboard as well as the internal USB translator module seen above. That translator box, called the RainShadow, is a PIC 18F87J50 controlled board that translates incoming commands from the USB connection and sends them out as CEC hex codes. A bit of code writing and [Valkyrie-MT] is in business. You can see in the video after the break that it’s not just controlling audio, he can now control the entire entertainment center including turning on the TV and setting it to the appropriate input.

From Wikipedia:

Consumer Electronics Control (CEC) is a one-wire bidirectional serial bus that uses the industry-standard AV.link protocol to perform remote control functions. CEC wiring is mandatory, although implementation of CEC in a product is optional The feature is designed to allow the user to command and control multiple CEC-enabled boxes with one remote control and for individual CEC-enabled devices to command and control each other without user intervention.

Trade names for CEC are Anynet (Samsung); Aquos Link (Sharp); BRAVIA Sync (Sony); HDMI-CEC (Hitachi); Kuro Link (Pioneer); CE-Link and Regza Link (Toshiba); RIHD (Remote Interactive over HDMI) (Onkyo); SimpLink (LG); HDAVI Control, EZ-Sync, and VIERA Link (Panasonic); EasyLink (Philips); and NetCommand for HDMI (Mitsubishi)

HDMI Connector Pinout

Pin Number	Signal Name	Pin Number	Signal Name
1	TMDS Data 2+	20	SHELL
2	TMDS Data 2 Shield	19	Hot Plug Detect
3	TMDS Data 2-	18	+5V Power
4	TMDS Data 1+	17	Ground
5	TMDS Data 1 Shield	16	DDC Data
6	TMDS Data 1-	15	DDC Clock
7	TMDS Data 0+	14	No Connect
8	TMDS Data 0 Shield	13	CEC
9	TMDS Data 0-	12	TMDS Clock-
10	TMDS Clock+	11	TMDS Clock Shield

Update:
http://www.kwikwai.com/