Audible is one of the few, last remaining marquee apps that Windows Phone is lacking. The popular "spoken audio entertainment, information, and educational programming" (aka books on tape) application is currently on every platform but so far has received little attention from the Amazon.com owned company. The last we heard from them was in October where they were non-committal.
Though official "support" can be a finicky source for stories, if we are to believe them, they do have a Windows Phone app in development with no ETA available:
"... our app developers are working on a WP7 app, but no date has been announced as of yet."
So that's that. Wish we could say more, like it's right around the corner, but we'll just have to wait till more info comes forth. But hey, it sounds promising.
Source: Twitter, (Thanks, Patrick H., for the heads up!)