We were there second week of Sept last year, and thought it was very low crowds! Liked it so much we're going back this year the same time.
Just out of curiousity, I went on the WDW website this morning and checked availability for the dates we'll be there (Sept 8-16). I was getting nervous after reading the threads about August being totally full! Anyway, as of this morning every single resort has availability from Sept 8-16. So I think that's a good sign that it won't be super crowded!