Wife and I went the 2nd week of March in 2009 and then again 2010. Somewhat crowded (big ticket rides, space and splash, soarin', etc) but not too bad. Plenty of fast pass available. I think the main issue was a lot of kids on spring break then because easter was in March. This year easter is in april, so I think less crowded this year early March.
Weather was hit or miss. Last year it was great, the year earlier got very nippy at night, probably mid to high 40s. during the day it was in the 70s adn very nice. Not waterpark or pool weather, but perfect for the theme parks.
Enjoy!