I have had many discussions, seen many talks, read many blog posts about Usability...that all somehow devolve to issues of Accessibilty. While the two are intertwined in some ways, they are two very different sets of problems, and it bothers me that so many people seem to confuse them.
Case In Point: "AJAX Usability". You hear a lot about how AJAX is unusable. Sure, in the wrong hands, such a powerful tool can yield unusable results. But AJAX came about as a usability SOLUTION. Eliminating screen refreshes, intuitive drag and drop interfaces, natural-movement animation, subtle highlight effects, editing in place, live validation, sorting and filtering ... They were all created to give smoother, better cues and more responsive interfaces to the user. The most usable sites on the web today use AJAX heavily (hello, 37Signals!).
Why does the difference matter? Because you have to understand the problem before you can solve it. It's possible for someone to be a usability expert, but not know much at all about accessibility. And vice versa - I have seen some really accessible sites with terrible usability. Perhaps the need for accessibility trumps any argument for usability, but not always. That's where progressive enhancement comes in to play. (Screen readers also need to step up to the plate and deal with RIAs, because that's where the web is going.)
So maybe in dealing with one you often have to deal with the other, but hey, I'm a web developer, it's my job to obsess over semantics :)