A study at the University of Auckland [NZ] looked at the cases of 43 babies who suffered lack of oxygen. In 9 of those cases, the survey found, kids were resting in an infant carrier for some extended period of time, when they exhibited such symptoms as "blue,' 'scrunched up' and 'not breathing.'" [Yeah, watch out for that last one; it'll kill ya.]
The study also found that over half the mothers in these cases were smokers. No word on how many of the smokers' kids were in either the "scrunched up, not breathing in car seat" or "left in car seat while mommy runs into the Exxon for some smokes" categories, but I imagine there's a big overlap.
Car seats endanger sleeping babies' breathing: study [cbc via dt reader paul]