I spoke with the Audible folks but they claim the issue is with a rooted Nook Color, and more specifically as follows:

Apparently, when using autonooter, it winds up identifying the NC as a Droid Eris phone, and the Audible app isn't compatible with the Eris!

I think that Nooter 0.2 might not do this, or the issue may lie in how the Google apps are installed.

Whatever the case, the real issue is that Android is still very much a phone OS and until Google gets its act together and comes out with a Android tablet OS there will still be issues.
This was also commented on at ZDNet.com recently and if Google delays, Apple may eat their lunch regarding tablet OS's - any way just my humble $0.02.

For now, I'll just keep my Audible stuff on my IPOD touch but it's a real PITA to have to carry two devices.