It is absolutely real.
When our son was born, he had jaundice, so the doctors had us feed him some formula through a syringe for the first 2 days to lower the bilirubin levels.
Our son started sucking on the syringe, despite our best efforts to avoid this. As a result, he became extremely frustrated while attempting to breastfeed, as the milk was not coming fast enough for him. We had to use rubber nipples with pumped breastmilk after that, as he would simply go limp and give up within moments of latching on to the real nipple, because it was too much effort, and he was used to how fast the rubber nipples gave milk.
To this day, if the nipple gets clogged, our son tends to pitch a fit (the only time he ever complains about anything is when he is hungry).
The lactation consultants we spoke with, as well as several books we read, indicated that artificial nipples should be avoided in the first 4 weeks. However, after the 4 week mark, infants are usually comfortable enough with a real nipple that nipple confusion is rarely an issue.
It is also worth noting that whether nipple confusion affects any given child is largely related to that child's temperment. There's a pretty wide variety of attitudes newborns have towards feeding, from very aggressive, to reluctant, as well as different levels of persistence. A child who aggressively nurses, even when his mother is waiting for her milk to come in, is probably much less likely to suffer from nipple confusion than some other children.