Not sure the exact date it starts, but we normally go to Orlando during President's Week and Mardi Gras has always been going on at US when we were there. (We're going 2/15 - 2/22 in '03.) We were in one of the floats about 3 years ago. It was a blast!
I'm talking about Mardi Gras at Universal Studios. It really is a big deal over there. They have a nightly parade with floats, beads, coins. The serve cajun food and have nightly concerts (headliners on Friday & Saturday).
Big Disney Fan...I found the dates on the USF/IOA board for Mardi Gras. Unfortunately, it's starting AFTER Presidents Day...Fridays and Saturdays only from February 21 - April 5, 2003, then every day from April 11 - April 26, 2003. (30 Mardi Gras event days and 10 headline concerts!) If you get a chance to go, it is a lot of fun. Being in the parade that one year was the highlight of our trip!!