Excerpt from here:
Audible updates its iOS app, adds support for the iPad | TUAW - The Unofficial Apple Weblog
Besides support for all iPad models, the updated Audible app also includes the following improvements:
Improved Navigation: easier to user while on the go
Simplified start up: sign-in with your Amazon account
New Library filter: hide your finished titles from view
Downloads on the go: download up to three hours of audio over 3G/4G
I'm esp pleased by the last feature.