I have a similar situation. Am running the Dell 3008wfp 2560x1600 30" monitor, and cannot download or view HD movies from iTunes; can do so with standard definition movies. Took computer to a repair store I trust. They downloaded an HD movie from iTunes (one I wanted) onto my machine, but connected to an Asus monitor. Both of us were using a DVI cable. So they think the problem lies with the monitor. Although when connected to the Dell monitor, the Nvidia control panel says that the card and display are HDCP-compliant. But read on...
Using:
twin Nvidia GeForce GTX 295 cards, each with two DVI ports, but no HDMI port
dual link DVI cable
Dell 3008 wfp monitor
updated drivers, along with updated iTunes and QuickTime player.
On another forum, someone else was having a similar problem. After talking with Dell and eVGA, plus testing, he came to the conclusion that this monitor does not run HDCP with DVI cable, despite Dell's assertion to the contrary. It will run HDCP with HDMI and DisplayPort. I only recently got into HD movies from iTunes, (or am trying to); the monitor has done well prior to this. I hope a solution does not involve equipment replacement, but it may.