In my opinion a prototype is used to prove there is a market for your product and attract seed investment.<p>An MVP is then created with the help of that seed investment and launched to help greater substantiate your product and attract further funding.<p>The Dropbox video is a good idea.  It may not be a prototype but it created enough hype for them to show investors that there was a market and demand for what they were building.  My only concern is that if you are creating a product in a new market you are potentially giving away a lot of your first mover advantage with that tactic.