this can be a problem, particularly in cradle carries in pouches and ring slings. it's worst in 'bag' type slings that are usually available at big box commercial stores where the only option is a low cradle carry. i know for sure of at least one baby who died in a pouch, i think it was at a LLL meeting - truly awful.
carrying baby upright, tummy-to-tummy is the safest way with baby's head just under your chin. then you can hear baby's breath, and keep baby's head in a safe position. if you want to do a cradle carry, a rolled-up receiving blanket can be useful in maintaining a safe position, and using a see-through high quality mexican rebozo or solarveil, mesh or similar fabric that baby can breathe through is a good idea. or a partial cradle-carry, where the baby is more upright but still partially reclined. i personally only cradle carry to nurse and then put baby back T2T, i don't like having baby's head too far away.
there's a mama over on thebabywearer who actually got hold of some equipment and did tests on oxygen levels with babies in different positions in different types of slings which you might be interested in. don't have time to find the link right now...
oh, and i want to note, too, that babies also can have this problem in carseats that are not at the proper incline angle, and in strollers and bouncy seats and swings. and those situations can be much more dangerous because the mama isn't right there and able to observe changes in breathing patterns. it's something people need to be aware of with newborns on a whole, not just with baby carriers.