great topic Dcrux
The way I always understood it...
accessibility is creating a page/site that is accessible to everyone (most focus is on visual and auditory impairments)
usability is creating a site that is easy to use (large focus on content layout, work flow and navigation)
usability and accessibility are functions of development they are technical aspects dealing with coding and layout of the site.
design is another piece (80% the won't really work without the other 20% tech stuff to back it up call it architecture if you like). design is a function of marketing, picking the right colors, font styles, where to place whitespace, content etc. etc.
Granted there are parts of the use / access - ability equation that are facilitated via marketing also but you always have to remember that no one will buy half a pie from the store..
Just my 2cents..