other than the cost and single use issues, both of which would reduce massivly if it became the vogue.
My thoughts would be focussed on fail-safe...what type of movement does it take to set the helmet off. Car air bags are deployed by the actions of a number of sensors and actuators. I'm guessing these are deployed by some relatively light weigh accelerometers or such. With this in mind, could you set it off by sneezing or looking suddenly to your right, or dropping off a kerb (perhaps that would actually be a great deterrent).
Additionally, as mentioned above, is this a rapid inflation-deflation device? if so there is a risk from secondary impact that the helmet cant handle. Car bonnets are becomming relativley soft places to hit...concrete is always gong to be hard. I wouldn't want the lid to deflate by the time the victim bounced to the floor.
Its got a feeling of foggy-ness about it to me. Car air bags react to a very distinct movement (trajectory) of a driver/passenger. the path is predictable and defined by the design of the container (the vehicle). On a bike, you can be thrown any manner of ways for any manner of distance and time.
On the plus side the area of protection provided seems far greater that a simple skinny lid.
Motorcycles have had a similar neck brace airbag for a while, if I recall.