Audible.com downloads: there's an app for that, just not on the iPhone.
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).
According to Audible, Audible Format #2 is "AM radio" quality, and about 3.7MB per hour. Audible Format #3 is "FM radio" quality, and about 7.2MB per hour. Format #4 is "MP3" quality, and about 14.4MB per hour. Audible Enhanced is "CD quality," at 28.8MB per hour. The iPhone is capable of playing back all of these formats, but the file sizes start to get big quickly. For example, the book I am currently reading (err, "listening to") is Bite Me by Christopher Moore. It's a 302 page novel which translates to just over 8 hours of audio. The Format #2 version is only 31MB, but the Format #3 is 58MB, Format #4 is 116MB, and the "Enhanced" version weighs in at a whopping 232MB.
To get around AT&T's imposed 20MB restriction, Audible would have to break up even the smallest format (#2) into two "chunks," and you would have to download them separately. It has been several years since I used it, but I believe the AudibleAir program for the Treo used to do exactly that. Sprint (my carrier at the time) did not have a megabyte-per-download restriction, and I once pulled over and downloaded a new Audible.com book while I was on a car trip. This was over the equivalent of Sprint's "Edge" network, but AudibleAir let me download the file while I was listening to the book, or segment, that I had already downloaded. Of course, it was plenty fast.
One of the many (MANY!) advantages that the iPhone has over my old Treo is built-in Wi-Fi, meaning that I could pop into a McDonald's or a Starbucks and download a new book right over the air, directly to my iPhone... if Apple would allow it. The rumor mill is suggesting that background processing for 3rd-party apps may be included in the iPhone OS 4, but we heard that one last year too and, so far, it hasn't come to fruition.
Trust me when I say that no one wants me to start writing a series of articles on the ways in which my iPhone is better than my Treo. The iPhone is significantly better in nearly every conceivable way. That said, I do hope that one day soon we'll see some variation of AudibleAir on the iPhone. However, in order for that to happen, Apple has to make some changes. Oh, and let's not forget that Apple would have to be willing to accept competition for audiobooks on the iPhone, since you can buy audiobooks through iTunes already. I certainly wouldn't expect Apple to do something as anti-competitive as blocking Audible.com in the hopes of driving more business to their audiobook store.