After upgrading to AppleTV 2.4 and installing Perian, I no longer can play Apple videos. The device just shows a black screen. Sometimes there is sound.
Edit: found a fix from westont on AwkwardTV:
Sure enough, looking in the /System/Library/QuickTime folder I found a ‘Disabled’ folder had been created. This folder contained the ‘QuickTimeH264.component’ file which had clearly been moved there by something. Moving this file back to the /System/Library/QuickTime folder and rebooting the ATV got everything working again.