Usually with expedia when it says there is availability, you can book. With travelocity, often it shows you a room and a rate, but when you get to the very end of the process, it says something to effect of "sorry, you have to call us."
My experience with expedia has been different from a poster above. Often when disney announces a room only code, you will see a corresponding rate on expedia -- usually with a different name, like "fall savings" or something like that. You can get these rooms from expedia even when WDW says its sold out. I've personally done this, and it worked like a charm. So I disagree that you can't beat AAA rates with expedia.
I believe expedia purchases options to buy rooms, not actual rooms. In other words, it pays disney for the right to sell the room, but can also put the room back to disney.
Every time I've made a reservation through expedia for a disney room, I had to prepay, but the cancelation policy was the same as WDW -- 6 days, althought expedia did charge some nominal cancelation fee, of like $25.
I have never been treated any differently at WDW when using an expedia reservation, and in fact have been upgraded. I think it's just like you made the reservation through any
travel agent. The problem is that CR won't let you put special requests on the reservation, you have to do that through expedia. I would stick with room-only with expedia and stay away from the packages.