I don't think the writers get a lot of say in it. During the WGA strike, there were a lot of blog posts about the industry, including that writers get stuck with a lot of restrictions on "controversial" topics, which I would assume includes "disabilities that aren't fixable tragedies."
They have the occasional blind character or person in a wheelchair, but never anyone who is only sometimes mobility-impaired, or can stand & walk fine but needs a special type of chair to sit, and of course, anyone with cognitive disabilities or serious communication problems can't be a star character.
I love the way "Malcolm in the Middle" deals with Stevie... he's seriously disabled in a number of ways, and Malcolm's family treats him like just another kid. (Well, Malcolm does. The rest of his family is erratic, but they're nowhere near Stevie's parents' hyperprotectiveness.)
Hmmm... has anyone put up a nifty master list of shows with recurring characters with disabilities?