The smallest unit I'm aware of that is on the market is the FRED Easyport AED, made by a Swiss company, Schiller. It is not approved for use in the US yet by the FDA.
http://www.youtube.com/watch?v=D95hN1CG90Q
Thank you for that, I'm trying to get AEDs approved to be carried in passenger elevators like the emergency phones are now - elevAED.com.
Concept: what would you have to add to an iPad (or iPhone) to turn it into an AED? The cables and pads, obviously, but I suspect the battery is close to functional, you have a nice display, strong processor - would adding a dedicated circuit do it? Touch screen defibrillation- why not?
Then all you'd need is to carry around the cables as a "distinctive" bolo-tie with amulet (pads).