There are very, very few things I miss about my Treo, but one of the few is my favorite app: AudibleAir.
AudibleAir is an application from Audible.com that would download audio books from your Audible Library directly to your mobile phone. It is available on just about every popular smartphone you can think of except for – you guessed it – the iPhone. The reason is because of the iPhone SDK limitations, as Audible explains in their FAQ:
"Unfortunately, the iPhone Software Development Kit does not currently allow third party applications the ability to wirelessly add content to the iTunes application. We are in discussions with our partners at Apple, and hope to be able to develop an optimal Audible iPhone experience soon."
If Audible could download their content and then add it to the regular iTunes library, you could listen to it through the regular iPod software. However, Apple says they can't do that.
Audible could make an application to download their content to their own app, and then let you listen to that content even if you left the application. Of course, Apple says they can't do that either, as it would require a background to continue.
So, Audible would have to create their own app to download content. That app would also have to play the audiobook, and you couldn't do anything else while it was playing.
Even if Apple lifted one, or both, of those limitations, there would be another hurdle. AT&T does not allow for downloads over 20MB. Audible currently offers 4 formats: the first three are numbered 2, 3, and 4; and the last is called "Audible Enhanced Audio" (yes, there used to be a format "1" but it has either been discontinued or, at least, deprecated).