I just got an e-mail containing the following information:
Effective immediately, Penguin Group has suspended the availability of download audiobook titles for library purchase across all vendors. This change does not affect any Penguin audiobook titles currently in your library's catalog. Your library will be able to purchase additional copies of titles released before 11/14/2011. However, titles released after this date and new releases will not be available, per instruction from the publisher.
If you have questions or comments, please contact Erica Glass at Penguin Group directly: Erica.firstname.lastname@example.org.
I'm not sure if this is related to Penguin's recent issues with Kindle books.