We were there in mid-April this year and the weather was great. Not too hot, but warm enough to swim. As for crowds, it really depends on when Easter is. The weekend of Easter and the week after things get very crowded, with a noticable drop-off the weekend after easter. So late April in '07 should be okay, especially if you get to the parks early and hit the major rides when you get there.
Get thee over to Tour Guide Mike, listen to what he has to say, and you'll have more time than you know what to do with. Even during the most crowded days.