I've never heard of it before, but apparently there has been an increase in flat head syndrome since the 1990s. It's officially called positional plagiocephaly, and you get it from extended time with your head against a flat surface while your skull is still soft. Think strollers, car seats, and sleeping on the back.
This study looked at 474 babies from 6 to 12 months. The ones with flat spots on their skulls (noticeable enough that they had been diagnosed with flat-head syndrome) had lower scores on cognitive observational tests, but their biggest deficit was large motor skills, such as rolling from the back to the side or lifting the chest off the floor while on the tummy.
They're going to check the same kids at 18 and 36 months, to see if it persists.
I guess what child development people need to take from this is that if you see a kid with a big flat spot on the back of the head, talk to the parents about a screening.