I also was disapointed that the D503 did not play video.
But as suggested above I was able to make the model change in the service menu and now it plays my videos (divx, avi, mpg, mov, mkv) on the usb!
Here are step by step instructions on how I did this:
1. Access the service menu by pressing (in succession) Info, Menu, Mute, and then Power when the TV is in stand-by.
2. In the Service Menu select Option , then arrow down to Model and select.
3. Then select model D5005 by using left/right arrow keys.
4. Exit by pressing return key twice, power off then on again. (did not do a factory reset)
The TV should have video playback from usb.
Hope this helps out.