I've seen some pictures

 

I believe the sensor in the seats started in 06...no?

 

people put kids on motorcycles w/o helmets too. Still not the greatest idea.

 

I have nephews one is 8 and the other 11. They have both ridden in my car. I am more cautious with them in the car, but I know that doesn't include what idiots are out there driving. I am sure you will make the best decision for your nephew.

 

True, is like on bikes: you have to watch for other a*sho*es driving like idiots, most of the time is not even your fault!!

 

I just did a quick web search and culled this from the CDC, NHTSA and American Academy of Pediatrics.
- Airbags pose a danger to kids and small folks only if they are not properly restrained and/or are sitting too close to the airbag when it deploys.
- Lap belts should be snug across the thighs, shoulder belts across the chest. This may require folks under 4'10" to use a booster of some sort to achieve proper restraint.
- The seat should also be back as far as possible to create distance between the undeployed airbag and the person. This is so their head doesn't impact the airbag when it's only partially inflated (snapping their head back). This is particularly a problem with short drivers who have to be an arm's reach from the steering wheel-mounted airbag, while passenger seats can be well back of the dash-mounted airbag.
- NEVER use a rear-facing child seat in an airbag seat.
- Everyone (including kids) is always safer in the rear seats, especially in the middle.

My thoughts: Broad rules such as "no kids in the front seat of an S2000" are put in place by manufacturers because they cannot control their customers' proper seatbelt usage and seat position. My daughter was 7 when I got my '05 S2000 and she road with me in a booster with the seat all the way back. Her soccer coach is a CHP officer (usually flies the plane around the SF Bay Area...some of you may know his work...) and in a previous discussion he agreed this was fine. Improperly restrained kids remain a big problem, and injuries can get compounded when there's airbags involved. Restrained properly, kids in boosters with the seats all the way back are safer with airbags due to the added protection. Anyway, my thoughts.