we thought this about September this year. We found it tobe awful although others didn't. Normally we don't plan what parks we visit and just decide on the day by what ever bus turns up..but we did end up looking at WDW Touring plans..but it still seemed extremely busy even doing this.
Never mind....back to Novembers from now on.
This is often the primary reason for such varied reports by different guests over the same time period. This, and simply comparing apples with apples. We started off going over Easter a couple of times and didn't know any better to think it was busy or not as we had nothing to compare it to. I used touringplans and followed them to the letter the first year and added more flexibility the second year. Then we went in October and first thing I wondered was whether I had missed some emergency evacuation alarm .... WHERE were all the people?!

The following year we went late September and stayed through the first week and a half of October and thought it significantly quieter than going mid October. This year we did the beginning of September and again found ourselves in the quietest parks we'd ever experienced.
However reports are fairly consistent that it has been busier this year (and last year in fact) than in previous years. I think a number of factors contributed to this:
The Epcot 30th Anniversary this year
(Last year was the MK's 40th)
More Disney discount and free dining promotions. Although relatively consistent with offers for UK guests, there seem to have been far fewer blackout dates for those booking through the Disney in the US.
Add to this the ever increasing flight prices, I do think more and more guests are taking advantage of the better flight deals.
I think the OP made a valid point in wondering if the "October secret is out" In my opinion, yes it is and even Disney is onto this now and using it as a major marketing tool.
As a side note on the subject, I doubt we'll be rushing to do September again. Quieter parks are lovely, but with fewer parades, shows and reduced hours and fewer guests in the parks I found it actually harder to get things done and there just wasn't the same atmosphere