Last month I complained that it wasn't possible to download ebrary content directly to the iPad, but that's changed sometime in the intervening weeks. As outlined in this ebrary knowledgebase post, you'll first need to create an Adobe ID (you probably have one already if you've dealt with their DRM in the past), and also pre-install the free BlueFire reader. Interestingly, BlueFire is also available for Android, so I suspect this means you could download directly to Android devices as well...?
Thanks to @jolietjosh and @heidisteiner for clueing me in!