I have been to WDW in September several times, both before and after 9/11, and I can tell you that September is empty in the parks.
Late August or early September is when most kids in the US start the school year, so very few families plan trips for the last week or two of August or the month of September.
And the weay I figure it, if the 7 for 4 deal last year didn't make much of a difference in crowds (which it didn't), then the free dining plan is not going to make tens of thousands of people suddenly plan unexpected trips to WDW just because they would eat free! You still have to pay for airfare, hotel, and park tickets, even with the free dining plan.
Also factor in the terrible hurricane season that central Florida saw last year. Four hurricanes hit Orlando directly in the month of September, a record number, and to the rest of the country, that's scary enough to make them think twice about booking a WDW trip in September.
WDW should be deserted during the week of September 12, but it will be hot and humid, with afternoon thundershowers every day (they only last 30-60 minutes). You will need to plan for the heat.
September also has shorter park hours. This is because there are so few people there that it is not worth the extra expense to keep everything open till midnight, so AK usualy closes around 5-6pm, MK and MGM usually close around 7-8pm, and Epcot closes at 9. But with the lower crowds, you can still see everything in the parks without rushing.
) That offer was just too good to refuse! Free dining and lesser crowds!!! 
I am a little worried that the offer might attract more people but I think that time of year it won't affect it too much. I was acutally working in WDW during Sept. a few years ago. It was Aug '00 to Jan '01 (before and after 9/11). Before hand I can tell you the crowds were very light compared to other times of the year. After 9/11 the parks were empty due to the tragedy (no one wanted to fly with good reason). So my info is mostly based on the crowds before. But if you were to rate the crowds as 10 being the worst and most crowded and 1 being the least crowded, Christmas week (which is without a doubt the most crowded) would be 10. September on the otherhand (after seeing the parks after 9/11) would be a 2 or 3. That is just my scale though. Some Disney researchers say about a 4. With that said, 4 is the lowest I saw on the scale. If you want to get a good idea of the crowd levels though at different times, go to the website for the Unofficial Guide. It has some useful info/tips. Anyway I shall stop rambling. But I would tell your DH to go for it! 
