Personally, I would go in November. You could catch Food & Wine at the beginning of November. I really do not like the heat. We are going the first week in June, but it's not really my choice. We can't really take the kids out of school for a trip because they're older (oldest is a Junior in HS) so our options are pretty much spring break or summer. We went spring break March 2010 and weather was pretty COOL (jeans every day and jackets most of the time too... although it was warming up the day we left!). It was more crowded than when we went mid August 2009. But I personally prefer the cooler temps over smaller crowds (they were still very doable in March if you know what you're doing.) BUT... my DH did NOT like going in March, says it's just not right to NOT be wearing shorts and t-shirts every day in Florida. So he prefers summer. We also went this past July and I thought it was WAY too hot, so we didn't spend A LOT of time in the parks. (July was a kid-less trip, though, so it was easier to just take it easy and not spend a lot of time in the parks.)
The key to visiting WDW during the hotter months is to KEEP HYDRATED (you can get free ice water from any counter service places) and hit the parks EARLY, take breaks midday (resort rest/nap and/or swimming) and return in the evenings.
Also, when we have gone in the summer, we always take at least 2 showers a day. Helps us feel a little more refreshed.