Hi all,
We did early Sept 3x when we had preschoolers and didn't have to worry about school constraints. It is a great time to go! At least in prior years, we had no lines at almost all of the popular attractions and often rode things 3-4 times in a row. Its easy to get PS when you want and where you want. And yes, though the parks close earlier than in the summer months, there's still plenty to do with the Boardwalk and DTD.
Each time we've gone in early Sept, we've stayed over the Night of Joy concert which I think is the Sat evening after Labor Day. We've never attended, but I understand it to be a night of Christian music performances in the MK. MK will close early on that nite, so we usually just plan to hit another park that day. Also I think this year that Labor Day is slated for the Tom Joyner family reunion (read it on these boards) but I'm not sure if that affects park hours. As long as you take these two big events into consideration with your plans, you'll be ok. I think its a small tradeoff for having the parks to yourself during the days.
Oh but the other questionable factor is that its hurricane season - don't laugh, we were there in 1999 when MK closed for the first time in its history ( I think it was hurricane Gloria). Pre-DVC for us so we had a long day cooped up with a 1yr old and a 2yr old at Dixie Landings! But we returned in Sep 02 without any weather related problems.