Just got back on Sunday from a one-week trip. Wore shorts all week - didn't need long pants at all (and the one night I wore capris, to California Grill, they were too warm!). Didn't even need a long-sleeved layer for the air conditioning indoors - short-sleeved t-shirts worked great indoors, unless we got really soaked by the rain.
It's really hot and humid right now - every day last week, it was about 90 degrees and very humid. Plus it was really rainy (last Friday, we had eight or nine separate little rain showers!) so be prepared for rain. Bring extra shoes in case yours get soaked, and have a good sturdy poncho ready. We bought WDW ponchos this trip and they seemed to be made of flimsier material than in years past - one of them ripped at the neck. (A friend brought hers from 2005 and it was made of better material.) I wish I had brought my nice Eddie Bauer poncho that I bought at Target - it was much less flimsy.