I just bought a book from Audible.com and was disappointed to find out I needed a windows program to download it. However I found a way to download it without their program! Here is how...

First purchase the book you want. Then go to "My Library".

Click on "Stream" under "Download It" Save the .pl file.

Open the .pl file in the text editor. You will find something like this
<Title>Brisingr (Unabridged) Part 4</TITLE>
<ref href="http://cdl.audible.com/cgi-bin/aw_assemble_title_dynamic.aa/BK_LILI_000762dmp332.aa?stuff about your account here fas;dlfilasddk" />
<MOREINFO href="http://www.audible.com" />
<TITLE>Brisingr (Unabridged) Part 4</TITLE>
<AUTHOR>Christopher Paolini</AUTHOR>
Copy the URL,
http://cdl.audible.com/cgi-bin/aw_assemble_title_dynamic.aa/BK_LILI_000762dmp332.aa?stuff about your account here fas;dlfilasddk
Past it into FireFox and you will download the .aa file! It is that easy.

I don't think you can play the .aa files on Ubuntu but you should be able to play them on your mp3 player.

Good luck,