Triggering a video at startup like the "Welcome Video" on Mac OS X Snow Leopard
I don't know how Apple did it, but a common solution to 'one-time only' actions is to check for the presence of a special hidden file in the user's home directory before doing them. 

So in this case, you'd have a login script that checks if the file is there. If it is, do nothing. If it's not, play the video, then create the file.