Just to preface this, I'm coming from a Prius and I've had my model S for about a month now. But I'm really not sure why the gas pedal is the right place for regen. On the prius as you hit the brake pedal it uses regen first and if you call for more braking than regen can provide it keeps regen on and also applies the 'real' brakes. You hit one pedal to brake and no matter how hard or soft you hit the pedal the car gives you the max regen and also ensures that car is still slowing down as much as you want. With 'one pedal' driving then you get regen as you let off of the gas pedal, but it only provides limited stopping power. If you want to stop faster than regen allows you still need to hit the brake pedal. It's counter intuitive.
Putting regen on the gas pedal creates two issues. First, is an efficiency problem... it's most efficient in a car to coast, regen is nice in that it recovers some power that would otherwise be lost by braking, but a decent amount of power is lost to heat as it gets transferred back to the battery. If you see a red light coming up in the distance you will save the most energy by coasting (no power applied to the wheels and no regen happening) as soon as you see the red light... if the light turns green before you get there, then hit the gas again and you won't have lost any power to heat. If the light is still red when you get there then you will be going slower than if you kept applying power to the wheels and you will still need to brake, but you will lose much less energy than if you had to brake at a higher speed. Of course it's still possible to coast in a one pedal configuration, but you need to hold your foot on the pedal the whole time and keep it pressed down just enough to prevent regen from kicking in and still not provide power to the wheels. It's much easier if you can just keep your feet off of both pedals if you don't want to apply power to the wheels or regen.
The second issue I see is a psychological one. If people get used to driving with one pedal (especially if the one pedal can bring the car to a full stop) then I'm afraid that in an emergency situation people won't think to actually hit the brake pedal to stop the car faster. If you get used to lifting your foot off the gas pedal to stop the car and all of a sudden someone pulls out in front of you then your natural reaction is going to be to pull your foot off the gas pedal and not hit the brake and that's not going to slow down the car nearly as fast as stomping on the brake pedal. I'm not sure people like me, who have been driving for 25 years now will ever get out of the habit of stomping on the brake in emergencies, but I think this could be a real problem for new drivers who learn on a 'one pedal' car and rarely, if ever, actually use the 'real' brake pedal.
I think having one pedal to go and one pedal to stop makes much more sense.