I am at Disney right now, in a
scooter. I have been here for 9 days now and gone to all the parks and most of the resorts (to check out Christmas decorations). Some areas have the painted box, some don't. However, nowhere have I seen signs that say the handicapped must go through the main lines. I just drive up to the curb at approximately where the back door will be, and I have always been loaded first with no comment. No-one has told me I should have gone through the line.
There may be a difference because this is a non busy time of year. Only twice have I been on a full bus.
But the signs that others have reported seem to have disappeared. I suspect the mainstream line concept has been abandoned because it just didn't work.
I have never seen any signs at the parks that said to go into the Mainstream lines for buses. The problem at the parks is that there are no instructions whatsoever about boarding the buses. In addition, the wheelchair symbols at the park bus stops to get back to the resorts have been removed, so it is not clear exactly what is expected.
After using the new Mainstream bus lines, I think the reason for making them wider was more so that people with huge strollers would be able to get through. Many of the strollers we saw were considerably wider than DDs wheelchair and barely fit through the wider lines.
The picture is much clearer at the resorts:
At all the OKW bus stops, there is a box like this painted on the ground.
Click for bigger picture:
The box is 30 inches wide by 48 inches long and is the fit box which is used to see whether an
ECV will fit on the bus. There are very few that wont fit into that box - many that seem too long will fit with a backpack removed.
All the OKW bus stops also have this sign, which says that
ECvs and wheelchairs, plus their party of up to 5, will be loaded first before other guests.
click for bigger picture:
As far as I know, most of the other resorts have similar boxes and signs at their bus stops too. I have personally seen the them at OKW, SSR, POR, POFQ, BW, BC, CBR.
All buses can load 2 ECVs/wheelchairs and some can load 3, if they are all in working order and there is room when you get to the bus stop.
For people with more than one ECV/wheelchair, its important to plan in case you dont all get on the same bus, so be prepared with who will go first, who will go with who and where you will meet up in case you cant all get on the same bus. It should not happen, but better to be prepared than surprised.
To leave the resort, position yourself close to the painted box, which will place you near the back door of the bus. When the bus drives up, let the driver know whether you want that bus or not (get eye contact and shake head for not wanting it - nod, wave, smile, what ever if you do want that bus.). On our last trip, we did see someone almost get left by the bus because they made no effort to let the driver know. The man was sitting on a bench and the woman was on the scooter, but not paying attention. All the other passengers were loaded for that stop and the bus was about to pull out when the woman kind of waved and people on the bus told the driver they thought she wanted to be loaded.
For leaving the parks, the bus stops for all resorts previously had a marked wheelchair waiting area/gate. The lines for return to all resorts are being made wider and the marked wheelchair areas are being removed.
Our last trip to OKW ended October 30, 2011 and I can tell you that ALL of the OKW bus stops at the parks have been changed and no longer have any marked wheelchair waiting area.
This is a picture of the OKW EPCOT bus area.
and a link to a larger picture:
Once you enter the line, there is not an exit until you get to the very front, by the front door. The OKW EPCOT stop had an area to the left of the picture where you could possibly wait with the ECVs.
The OKW stop at the Studio had chains that prevented doing that and kept everyone going into the chained lines of queue.
We were not there at a particularly busy time and did leave at non-peak times, so probably had a better time getting the buses. We were in the first 10 waiting for the bus each time (and often were the first to arrive at the stop).
If you try to avoid the peak travel times, you will have lots less trouble getting on the buses.
We did some travel from a park to a different resort, so did see some of the other bus stops going from park to resort.
There is no consistency. Some were metal bars like the OKW picture.
Some had chains with hooks that could be be easily disconnected.
Some had chains without any hooks, so could not be opened.
Some had part of the queue with chains and part with ropes that had hooks.
So, it is very much shades of gray and not much black and white at this point.
We ended up scrapping our morning at AK because we couldn't put the kids on the bus again.
Now if those 5 out of 6 were bad luck or more like amazing luck considering the numbers of them out there compared to the new kneel down ones...... who's to say? 
