March is hit or miss, mainly based on wht school districts have the week off.
This year, Easter Sunday is April 16th, and Disney has traditionally offered Peak scheduling the week before and the week after Easter, so April 7th thru 23rd in 2006.
But many school districts are switching to a "spring break", which falls on a certain time frame of the semester (say mid-point, or 6 weeks after the semester starts, for example). This causes many folks to show up in March, some years the 2nd week is slow, but the 1st and 4th week were busy, and the third week was very busy.
It is really hard to predict, but Disney has scheduled 10 PM closings for Disneyland on weekdays in March starting the 13th, showing the fact that they expect larger than normal crowds.....