We went on March 10th of this year and the weather was great. Comfy during the day and wore a jacket at night. Some schools have spring break at that time but didn't notice any huge increase in crowds at WDW.
Might also want to check Mardi Gras dates in Louisiana. Sorry, I don't remember when it is in '07. Lots of folks take WDW trips since the kids are out of school for several days.
My niece and I went one year during that time and it was more crowded that I thought it would be.