Some of Apple iTune's video content is being wrapped in HDCP copy protection that makes it impossible to output the movies on any non-HDCP screen. That's a pain in the ass, but not unlike Blu-ray and other HDCP-compliant video sources that have been around for the last few years.
Still, this is the first time that Apple has added HDCP restrictions to the standard external monitor port of its hardware. What a rip.
Apple brings HDCP to a new aluminum MacBook near you [Ars Technica]