When pressing I, I now receive:
keying set failed to insert any keyframes.
No idea why pressing I doesn't work anymore. What did I do wrong? (I've upgraded from 2.71 to 2.72 and it stops working).
See screenshot: The solution was to remove the keying set at the bottom and when selecting the camera, to remove the 2 keying sets that were there.
I however have no idea how this was enabled, and at this point of time I also don't know the goal of this keying.
After removal, the I key worked again.
You may have locked the editablity of the keyframes for the channel. This means that the little lock icon next to LocRot in the graph editor was pressed. Atleast this is how I've resolved the issue on my laptop :)