Did you see this news story out of the UK? Apparently they are teaching children to read too early and it's going to permanently hinder them. The doctor recommends refraining from "exposing" children to the alphabet until age 5 and a half.
It's not a complete enough story to really get a clear evaluation but it seems to assume that all children develop and learn to read at the same age. I come from a family of three children. Sibling-the-Elder learned to read at three. Sibling-the-Younger at five. I being the middle child that I was, started at four. All of us are now avid readers--which probably has far more due to the fact that the Incredibly-Patient-Mother read to us and with us and encouraged us to read rather than the age at which she started teaching us our ABCs.
I'm not saying we should force literacy down toddlers' throats, but there is no reason not to start to encourage early literacy traits. And if read to and exposed to books regularly (twenty minutes sound familiar anyone?), many children will start pre-reading before the age of six. If I can learn cursive at six, is it so much to ask that a four year old be taught to begin to write his/her name?
It would be interesting to see more on this...particularly the study where Scandinavian children are proven to be better readers for starting later.