I've been researching this myself recently and have found a number of options the choice of which will depend on your level of technical expertise, convenience requirements and software availability/stability. In no particular order:

1. Burn and rip CD's - Technically this is reasonably straightforward if time consuming. If you can burn to an ISO file it will be quicker and save you media.

2. Capture player output from soundcard. After the initial setup this is again reasonably straightforward, but as it happens in realtime it takes longer than actually listening to the book. E.g. A seven hour book will have to play for seven hours and then you might need another hour to split the file up and save it to mp3. I did hear on the TWiT podcast of someone doing this for their entire library using some scripts on three computers running continuously for about two weeks.

3. Use a software converter. It's a cat and mouse game so may or may not be working with the current version. Have a look at this thread on Afterdawn: http://forums.afterdawn.com/thread_view.cfm/23/103313

4. Run the Palm (Treo 650) Audible client on the Garnet VM. This method currently has the best potential, but the Garnet VM is not stable enough for long term use. I did manage to install the Audible player, but the VM crashed and lost the setup before I could get and books on to test if it worked.

5. Use the Audible Java player if a Maemo J2ME vm is ever released. There has been some work on compiling java for Maemo, but only for console apps. If a Mobile Edition GUI is ever release then this might be a solution.

6. Keep emailing Audible to ask for a native player. Try audible@custhelp.com and http://www.audible.com/contactus. Here is the lame response I got:
I plan to email them back pointing out that the OS has nothing to do file format support and that with the Google Android platform coming next year, the number of Linux based devices is only going to increase.
On the subject of Linux support. The Audible Windows desktop client has been run under WINE with some success. A quick Google will find you details. This only works with x86 processors though so no Maemo.