In 5.5 the option to select a frame in a video and have it displayed on a page in an ePub prior to playing the video did not work. The code required to call an image for a video was not exported from inDesign to an ePub document. The only way to remedy this was to open the ePub document and add the image into the image folder and then add the missing code - re-zip the fie and rename it epub.
I would like to know if this issue was addressed in version 6.
It worked for me with InDesign CS6 when I tested on my iPad.