Apple cinema display 23'' arent HDCP complient...sold in <2003 if im right.
Youre Gpu card is, and its the one that is sending HDCP signal to the ACD23.
HD movies in iTunes are touched by the HDCP security and this is why you got f0ck***
Try to play the movies thru youre ibook plug into the ACD, it should works.
Or you could get a crappy Gpu like the 7300gt (macpro2006) I think isnt HDCP and hook the screen to this card when playing HD content that are HDCP (cpu will handle up to 1080p even on a 7300gt)
Intel are to blame. I think HDCP IS A SCAM . Not Apple...they are obligged to provide their store HDMOVIES in HDCP .
Plug any blueray drive or anything playing HDCP, same result.
HDCP SECURITY: source: http://en.m.wikipedia.org/wiki/High-bandwidth_Digital_Content_Protection
Thank you very much Mikael, for your very helpful response. I got a load of HD content last year, when I was still on my trusty G5 Powermac (using the same screens - 2005 model). Everything worked fine but I'm now on a macpro and my HD content has been disabled - what progress!!! At least I now know what's happened - it truly *****!!!!!! What really annoys me is that the iTunes store still lets me buy content that my my machine can't play - that's just wrong, as I now find out that it's supposed to disable the purchasing of protected content if the computer doesn't support it!
Anyway, at least I can play it on my MBP!