As you can see in my siggie below we have taken both our trips in Sept. I would recommend somewhere around my 2008 dates if you don't care about Haunted Mansion or Space Mountain. They are closed then for holiday overlays. The second set of dates was more crowded (could have just been 2009 compared to 2008) but we also ended on the launch of Halloweentime which made the parks much busier on that last day (and even the days leading up). You do make a big sacrafice with the entertainment schedule and park hours but for us the less wait time makes up for it. Last year it was actually quite hot for Sept (hit the low 90s on 2 of our days) but in 2008 the weather was beautiful. No rain on either trip. Things to be aware of when planning in Sept are: 1) Labor Day, 2) Marathon weekend, 3) conventions (check the anaheim convention center calendar), 4) Homeschooler's week, 5) Halloweentime debut