Check out the Theme Park Strategy Boards.
I booked early-mid March thinking Spring Break was at the end of March. I remember reading a lot of sites and forums including Dis) about when busy periods were. Spring Break period when the US schools/colleges break up before Easter being one of the busiest times of the year. Some have said busier than mid-August. Now this I find a little hard to believe, but I did notice this being said a few times.
I have been told very early March is okay, but gets busier each week. I'm back on 17th March, so hopefully will miss most of the crowds. Spring Break is supposed to be a bit of a nightmare. While not quite on the scale of our Chavs/Neds, the US teen like to let loose when college breaks.
Happy to be wrong here