Went in May '04 and '05 and going May '07---the weather is perfect! You can go to the weather channel website to look up the averages---all I know is it is before the rain season (June) or hurricane and is warm to hot---but not nearly as hot as it can get in teh summer---they say May and Sept. are the best months to go to dis weather wise----one afternoon we had 1 hour of rain other wise, we expereinced 8 days and 6 days of pure perfection!!
Seemed like it was low- to mid-80's every day when we went----AK seemed hotter than the other parks---maybe it's the rainforest folliage there?
Pack one sweatshirt each in case it gets cold at night---we only used ours one nioght when we took the boat to HDDR (or just buy some if you need them at wdw), pack cheap rain ponchos, and pack only shorts, and light/sleeveless tops and you'll be all set!
When we swam at night in the pools, but I did feel a tinge of a chill, but I like the HEAT! Others found it to be perfect swimming weather.
Hope this helps!