Well, first thing to note is that September will still be very hot. However, September is still very much hurricane season and you will find it is very similar to June/July/August in the typical Orlando weather during those months i.e. hot, cloudy and very wet, mixed in with a few brighter days (although still daily thunderstorms). We've been before in September, and thought we might miss the worst of the weather going a bit later in the year but we were wrong. In fact, when we left Orlando to fly to New York for our 4 nights there, it was touch and go weather any flights were going to be going from MCO at all, the weather was that bad. It's certainly nowhere near the absolute best time to go in my opinion.
Personally, I think June/July/August/September should be avoided if at all possible, although of course as has been mentioned on the thread by Karen recently, a holiday to Orlando at anytime is still better than none. May is better, however this year it was very wet, which certainly goes against my experience of far better weather than the months mentioned already.
My favourite time of the year to visit Orlando is end October/early November (remember Thanksgiving in November, you don't want to be there then). This obviously necessitates a long wait for your major holiday of the year, but it will be worth it. We had 11 nights during that period in 2006, and only on the final day did we see any rain (quite heavy rain to be honest). Up until that point we had experienced temps between 24-28c, clear perfect blue skies, and glorious sunshine. It was a pleasure to visit at that time, warm enough (t-shirt and shorts day and night) but none of this sticky, oppressive temps you get during the summer months and the lack of rain was a Godsend.
Factor in some great hotel deals, $39 a night + tax at both the Quality Inn Plaza and Comfort Inn LBV (both booked direct) and it was a cheap trip (we were already in the USA as part of a round the world trip anyway).
I've heard good things mentioned about the winter months, but obviously temps will be cooler so it might not suit your girlfriend as much? Late October/Early November still provides the warmth day and night which is what Orlando is all about really. Also, worth noting severe lack of crowds at this time also, longest wait 30mins for Men in Black (of all rides), and all the water rides including Splash Mountain basically walk straight on. Perfect.
HTH