HDCP shouldn't be an issue IMHO (should be checked though). You should receive a 480p signal if you can't negotiate encryption. 480p should be more than enough for an ambilight-ike solution.
My background is in programming so I would be very comfortable with a RGB matrix.... if we could recover a RGB signal from HDMI that would be optimal. A clean in the middle ambilight-like solution.
If you intend to work with HDMI, you should start by trying to understand the protocol and pin out of the cable
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PINS
http://www.hdmi.org/installers/insidehdmicable.aspx
HDMI - Wikipedia -
TMDS signaling
Transition-minimized differential signaling - Wikipedia
http://www.ramelectronics.net/tmds.aspx
You should be able to derive RGB information from each TMDS group. However, Arduino IMHO is not suitable for the job since it probably is not fast enough to process HDMI on your time constraint... no lag.
You could try this instead:
it seems to even handle HDCP so you would end up handling the much simpler VGA signaling. I am interested on following your progress.
Or, HDMI to component if that's what you prefer