I forgot to cite some sources:
Audio Return channel[1] Claims to be able to carry the signals normally routed over an S/PDIF cable from the video signal receiver (sink) back to the video signal transmitter (source).
Ethernet channel[2] Offers up to 100Mb/s bandwidth networking.
HDMI Specification Availability: "HDMI Specification Ver.1.4b and the Compliance Test Specification Ver.1.4b (CTS 1.4b), released on October 11, 2011, are available for download only by Licensed HDMI Adopters." [3]
"The HDMI Specification Ver. 1.3a is available at no charge. To download this version of the HDMI specification, please complete the form that follows.
The HDMI Specification Ver. 1.4a is available to HDMI Adopters via the Adopter extranet. Adopters can click here to log into the Adopter Extranet."
Adopter Registration: "Licensing Option
Please select which HDMI Adopter agreement is appropriate for your company.
Manufacturers of End Products that will include HDMI:
Ten thousand fixed annual licensing fee plus royalty Five thousand annual licensing fee plus $1.00 administration fee per licensed product plus royalty"
I'd love to download the specification but they have it behind a signed agreement + steep pay wall.
References:
[1] http://www.hdmi.org/manufacturer/hdmi_1_4/arc.aspx [2] http://www.hdmi.org/manufacturer/hdmi_1_4/hec.aspx [3] http://www.hdmi.org/manufacturer/specification.aspx [4] http://www.hdmi.org/manufacturer/adopter_registration.aspx