Sony Vegas 8.0 projects that use mp4v files may not display the video properly, presenting only audio with a black screen. It appears that QuickTime 7.6.8 causes the problems. In my case, the .mov files, which originated from a camera phone, would play fine in VLC, but not in Windows Media Player.
The solution is to uninstall QuickTime 7.6.8 and install QuickTime 7.6. Unfortunately, this does mean you lose the updated security features of 7.6.8; one hopes Apple rectifies the situation in a future release.