I just returned last night, and I think I can officially report Wednesday, 9/19, as the full arrival date.
Starting on the previous Saturday, I was seeing one or two pairs a day. Then on Wednesday, lots of them, but not swarms. There were a whole bunch at the Grand Floridian - particularly the monorail station. (pay more, get more?) And the same day, there were quite a few at Animal Kingdom. (Maybe they like the white buildings at the Floridian?)
The odd thing is that they don't seem to be everywhere at the moment, just certain places - while there were lots at GF and DAK, there were a lot fewer at Magic Kingdom the next day, and not many at all at Epcot. There were enough to notice at Wilderness Lodge, but not many at all at the Boardwalk.
Summary - they're heeeeere, but they've not yet hit in full force.