There was a really good documentary about children's language development on BBC2's Horizon programme last night, called Why Do We Talk? Have a look here on BBC's i-player this week to watch it. And the BBC magazine has an article about it here.
And there's more on Deb Roy's massive child language data experiment here on a blog post back in July of this year. Meanwhile, have a look at this TED online lecture by the mighty Steven Pinker for his view on "the blank slate". But first, make yourself a nice cup of tea and put on your thinking cap (rather than your dodgy New Era hat).