Our last trip was 8/14-8/24/03 and the only day it did not rain at all was 8/17. Of the 9 days it did rain, I would say 5 were the quick storm variety that happened in the afternoon but cleared up in 30-60 minutes so you could do the parks in the evening. But 4 of the days it rained steadily for hours in the afternoon and/or evening, including 8/18 when it absolutely deluged in the evening while we were at Epcot to the point that there was at least a foot of water in places, especially between Wonders of Life and Universe of Energy.
If you go in the summer months, ASSUME it will rain every afternoon and/or evening. This way, you are prepared with ponchos, umbrellas, change of shoes, plastic cover for stroller, towels to dry yourself or seats, plastic bag or container to keep money/camera/passes dry, etc. And also keep a flexible attitude and have fun -- a soggy evening in Epcot with your feet underwater is still better than a day at work!