I think there may be a discussion of that in What to Expect... Something to the effect that if you leave your kid laying anywhere (car seat, crib bassinet) for 23 hours a day for the first 18 months, there is a risk of flat spots (if I'm exaggerating, it's not by much). But really, think about it, the alternative is to have the kid laying flat on his back on a crib, which is actually going to put more pressure on the back of the head (the angle of the car seat will reduce the pressure). That story shouldn't be interpreted as don't let you kid sleep in his car seat, but rather pick up your kid every now and then, wherever he sleeps.

Our pediatrician would always start off with questions about sleep habits, and would always seek to allay our concerns about sleeping in the car seat; he never once encouraged us to think about having him sleep elsewhere.