Seems I am having the same problem. My installation is the 7.10 release, and I chose the alpha blending GUI. Before, with RC3, I chose the old GUI and video was displayed correctly!
Don´t know if it is interesting: After installation I changed screen resolution to PAL 720x576, and modified xorg.conf manually for making the S-Video output work with my NVidia card.
Orbiter GUI works, MythTV works, Live TV and playback from MythTV works, too, but when playing video from the orbiter, the screen is just blank - but the onscreen orbiter´s overlay controls are displayed, for example when I pause playback from my laptop using the Windows orbiter.
Can anyone help?