Actually, when we were there in 2009 (free dining), the week before labor day was pretty light (walked on Expedition Everest multiple times in a row, for example), the weekend (Sat. and Sun.) was quite crowded, and Labor Day was DEAD. Very low crowds.
I think everyone was traveling that day to get back in time for school.
I'm actually shocked they are predicting an 8.
I think that most people with kids will be back to school, with some exceptions, of course. We meet a lot of other homeschoolers that week.
The limited sale COULD affect it, but I think most August travelers travel then because they do not have the option of Sept. The most you could get is the October or later travelers, and if something good comes out for them before Sept., they will probably change back to later in the Fall.
We've been to WDW during free dining for August and September dates, and never has it been very crowded.