We just got back from a late May trip after having gone three times in late February, and once in late November. We will probably try to avoid May in the future, as we thought (being Midwesterners too) that it was too @#$ hot! (upper 90's every day) I've never sweated so much at WDW in my life! Thank goodness the kids weren't with us, as we only stayed at the parks for short periods. Then we came back to cloudy, rainy Ohio where the temperature hasn't been above 70 since we got back!
February (late February) is our favorite - even though you run a chance of some rain. It is a good break from the bleak, cloudy, snowy, north and gives you a hint of spring. We have always worn shorts, and always gone swimming (you just have to get a towell quickly on some nights). As long as you stay away from actual Presidents Day weekend, the parks are not so crowded too.
November was neat too, as the parks were all decorated for the holidays. The only bummer with November is that you come home to many months more of winter!
We will be there again in late February 2002 also - and doing a 3 day Wonder too!