OK I have a theory about ADR's. Note this is just my speculation and is only based on observation - I have no inside knowledge.
Assume they have a pool of tables. I think they release a small percentage for early booking with 90+10. Then on 90 days out, they release another larger percentage - the majority of the tables. Then on a much closer date, like a week or less, they release some more tables. This would be done so that the people who didn't know to get ADR's ahead of time have a small chance to get a table. And finally, they probably reserve a small percentage for walkups, table adjustments, lost ADR's, etc.
I think they also adjust the number of tables in the pool based on the time of year. Based on past history, if they didn't fill the restaurants during the slow season, they would put less tables in the pool and plan on less staff. When they realize that the ADR's are all taken and it looks like they need more tables, they can add more in and plan on more staff. This reduction of staffing and ADR's can lead to no ADR available, but lots of empty tables - yes there are tables available, but no one to serve or cook for you.
Also for park restaurants you have the hours issue. They don't know what hours they will be open at MGM or MK (those are the two where the hours vary the most). So they don't want to book an ADR at 7pm and find out they are closing at 6pm. So they start with their core hours that would be safe no matter what. Then when they have more confidence in the hours, they extend the time slots, adding more tables available for ADR's. And this can be on either end - if things are slow, they wait to open for lunch. i.e. they would rather serve 1000 people in five hours than 1100 people in seven hours even though some of those 1000 people would rather eat during the outlying hours.
As for the ADR's going quicker, I do think it is the dining plan. Many people have said that by purchasing the dining plan, they are doing table service restaurants that they would never have done without it. The difference between this plan and the old dining plans is the cost - people will pay $35 /person/night, but not $60 (I don't remember the exact cost, but it was in this neighborhood I think) even though it gave you two table service per day. Most people didn't want two table service per day, every day. So many more people are buying the new plan versus the old plans. And by buying the plan, they are realizing they need to make ADR, because either their
travel agent or Disney or guide books or DIS or whereever has told them to do so. By buying the plan, they are thinking of their dining plans now rather than waiting until they are there or even until they are hungry.
They are raising the price for 2006 by $2.99/adult and $0.99/child (8.5% and 9.9% respectively) which will cause some people to not purchase it as the value is a bit less (assuming all restaurant do not raise their prices the same amount or more). Plus it might push it out of some people budget reach. So things might stablize next year, but I still expect many people to purchase the plan and the restaurant to remain full.