OK, I did a little experimentation last night.
I had been trying to connect the Nexus 7 to an "AUX" HDMI connector on the front of the X500, which is obviously desirable for a device that will normally be connected only temporarily to the receiver. Yesterday I tried turning on and connecting the devices in different sequences, but was not able to get video to my projector. I was getting audio temporarily through the receiver, which would be interrupted each time the receiver and projector failed and retried to connect.
Then I connected the Nexus to one of the HDMI ports on the back of the receiver. At first I was unsuccessful, but i found that if I switched the receiver to a different input, then switched it back to the Nexus HDMI port, suddenly it worked! This is less than ideal as I would much rather use the front port, but at least I have something that works now.
The moral of the story is that the HDCP handshake is a pretty dodgy system, very sensitive and easily disrupted. If you ask Denon, they'll say it's the fault of your TV or the SlimPort. If you ask your TV manufacturer they'll say it's Denon's fault or the SlimPort's. And so on. As a first step try every combination of HDMI ports, cables, equipment startup sequences and source switching sequences. Also, here is a link to a thread that says the handshake problem can often be solved by inserting an HDMI switch somewhere in the stream - worth a try if all else fails. Good luck!
http://www.avsforum.com/t/1096050/solution-to-hdmi-1080p-receiver-display-hdcp-handshake-problems