Steve Rubel points to a Gnomedex announcement - Audible is offering their audio content via secure (i.e., subscription) RSS:

They are making its wide selection of periodic audio content available via secure RSS. Audible customers can schedule automatic delivery of their periodic programming to their computers and to any one of more than 135 AudibleReady handheld devices.

BottomFeeder will deal with that nicely - you can use the Enclosure plugin to manage automatic downloads of the podcasts.