I have always said that it is never too early to read to your baby. Hubby and I ready a picture book to Little Girl every night while she was in my belly. I know, the pictures were lost on her, but we loved the belly time, and we thought it was a great way to instill a love of both reading and family time to our child before she (though we didn't know she was a girl yet!) arrived.
Once she was born, we kept up the reading. It is an amazing way to teach language, rhythms of speech, conversation skills, and lots of other valuable lessons. It is also great for those long stretches when a baby is really young and you don't know what you are "supposed to" say to him or her!
Here is one article from BabyCenter to help explain some of the benefits.
This is an article which discusses some of the joys and benefits of reading to your baby in utero. And this one from eHow has a bit more information you might be interested in.