I'm by no means an expert on the subject, but in trying to figure out the best way to book our June 09 trip, and several phone conversations with WDW and AAA, I can answer some of your questions ...
AAA discounts are only on the rooms, whether its a package or room only ressie, it is in fact only the room portion of the cost that is discounted. Also, if you do want a package, you have to book through AAA to get the AAA discount. WDW can only book AAA "room only" discounts. This doesn't entirely make sense because the only thing discounted is the room anyway, but for whatever reason, if its a package, must go through AAA.
Those discounts are pretty well locked in, depending on the time of year. Summer season for my trip next year is only a 10% discount on a moderate resort. You do get a higher discount in the value season and the discount varies for value - deluxe resorts.
If you book through WDW, not a TA, but actually WDW, you can have AAA take over the package and apply discounts. I've heard though that there are only so many rooms at a given resort that are eligible to receive the AAA discount so best to book early.
For us AAA is probably what we are going to use to book our trip since it is the only "guaranteed" discount I can find. We are on a budget and waiting to maybe get a code, or even an AP discount probably will not work for us this trip, though from what I've read, those can mean even greater discounts. But even a 10% discount off of 10 nights outweighs the annual membership of AAA.
Plus AAA is great for all the car related stuff too!