I'm not sure if it's just the newer iMacs from late 2012, or if older ones also had this issue, but the target display mode works VERY inconsistently on my 2012 iMac. Sometimes, it just works. Other times, I have to reboot BOTH my Retina Macbook Pro and the new iMac to get the target display mode working. Any ideas why this is happening? I have the RMBP hooked up to the iMac with a Thunderbolt cable obviously. And I press command F2 on the keyboard of my iMac after waking up the RMBP... Again, sometimes it just works.. other times I have to do complete reboot cycles.