While the movie is playing (I tested this while watching a transcoded movie on my Xbox 360)
Jump Forward - Right Trigger pull and release jumps the movie about 30 seconds forward. Pull and release repeatedly, quickly, and the movie will jump i guess 30 seconds for each pull. Hold the right trigger and the movie will end and return to 0:00 minutes (and will say done).
Jump Back - Left Trigger pull and release and the movie will briefly rewind (very briefly, not even thirty seconds). Pull and release repeatedly to move back more, but it still seems to be about 5 seconds jumps.
Pause & Play - Right Thumb Stick: click it like a button and it pauses the movie. Click it again and the movie will start playing again.
Show Progress Bar - Right or Left Bumper brings up the progress bar for the movie.
Move Back Through Menus - Back (<) button movies you backwards through the menus
Return to Main Menu - Start (>) button brings up the main media center menu
Show Movie Catalog - (B) button brings up the movie catalog
Show Keyboard - (Y) button brings up the keyboard interface
Details Menu - (X) button brings up Video Details, More..., Zoom, and Settings menu
(A) is used to make selections but doesn't seem to do anything while the movie is playing.
Direction Pad doesn't seem to do anything while the movie is playing
If I am missing something please let me know.