Hey Dany! Welcome to the boards!
As per everyone else that responded, you've selected one of the absolute BEST times to go. Especially since it's after Labor Day! I went Sept of '07, Sept of '09, and I am currently planning a trip for Sept of '11.
Now i should mention we do not have any school age children right now, so we have freedom of traveling any time of the year we would like.
Pros for Sept: Kids across country back in school, free dining if you so choose to partake, heat bearable (don't let me fool you though, it's still hot), Food and Wine Festival at Epcot, and finally if you go late enough in Sept, Mickey's Not So Scary Halloween Party... we did the latter for the 1st time last year and it was the best time we've ever had at WDW!