We have gone during June, July and August. I don't think there was any difference in temp. from one month to the next - they were all really hot! But as the previous posters have said, you can do things to beat the heat.
Drink lots of water, use the mister fans, and the most important for our family - take an afternoon break! We always leave around noon and head for the pool. We usually go back late afternoon, grab some dinner somewhere in the parks and stay till closing.
I think the most important thing to do to avoid the heat as much as possible is to plan, plan, plan. Plan when you'll take breaks, where you'll eat so you can stay cool, if you'll take a mid-day break, etc.
As far as lines go, make sure you use Fastpass. We take advantage of Early Entry (now Magic Hour, I guess) and hit all the really popular rides first. We save the less popular attractions for later in the morning or afternoon when everyone else is standing in line for 60 min or more to ride Space Mtn or something similar!
I don't know if attendance patterns will be the same as the first time Disney offered EE, but traditionally the park that had EE that day tended to get crowded early and crowds built even more during the day. We would use EE, then leave at noon, and go to a different park in the afternoon and always found it less crowded. I would think this would be the pattern again.
Hope this helps. Whenever you go, I know you'll have a wonderful time, heat or no heat!
