Phillip goes through Audible's podcasting support in great detail noting a great many of the issues with it.
I think I may have the same problem with the formats, especially if iTunes does this as well. Right now, I'm preparing for a day visit to Montreal and am loading my Pocket PC with podcasts. If I had downloaded an ACC file, I would be out of luck.
Podcasts as a distribution method need to be free (unless they ARE DRM'd - not the other way around). All of the companies offering this need to figure that out a bit better, if Phillip's take is any indication.
MAKE: Blog: Audible does Podcasts - the complete guide (and HOW TO)