Hhmm..i wonder how much side curtain air bags might mitigate some of the danger?
this is an issue i've voiced opinion about recently; watching side impact videos and videos or still shots from automotive websites of deployed side curtains, it seems to me that curtains would only be beneficial for older kids whose heads are at a height similar to an adult's head. the curtains generally don't deploy as far down as the window sill. here's a promo pic of a deployed SAC in a new ford:
on the other hand, this GMC curtain seems to extend much farther down, and may (major handwaiving here) mitigate the dangers of a child's head slumping in a carseat:
in addition, if you watch the booster video that jools posted, you can clearly see the dummy's head hitting the sill, not the glass area. so, just like seatbelts, side curtains seem to be, in general, optimized for average adult sized occupants, not small children. this is why i feel so strongly about SIP seats, lack of testing standards notwithstanding. i can't trust that the curtains in my car will be effective at protecting my small kids in a side impact.
i do agree with a previous PP that head slump in a harnessed seat is probably less dangerous than in a booster, since 1) the body is contained better in the seat, and 2) the seat is contained better in the vehicle (a properly installed harnessed seat will not slide as much as a booster held in loosely by the seatbelt).