Given that you are most likely to use the horn to prevent an collision, and that during collisions the airbag deploys.. I would say that the center of the wheel is just about the worst place to put the horn button.
Slapping yourself in the face somehow seems worse than the airbag deploying properly.
As someone who has had an airbag go off 'in my hand' (resulting in, among other things, a hyperflex of my wrist... trust me, the 'slap in your face' is the least of your concerns. The airbag will be doing that too.
