As of right now, it seems Microsoft and Audible haven't reached an agreement to allow the former's portable music players to play the latter's book-on-tape files. For those unfamiliar with the Zune (pictures and details here), it's a hard drive based music player from Microsoft that's launching this November. Audible is an online book store affiliated with Amazon, which only allows approved music players to play back their media files.
So far Microsoft hasn't gotten Audible's approval for their Zune, but they're still in talks. Even though we personally don't use Audible, we're sure it's a deal breaker for people who do. There's three months left until launch, so Microsoft has plenty of time to wave money in Audible's face to get them on board.