Offline-mode has gotten some press since it was added to the Firefox roadmap. Some of the rails programmers have probably been watching Joyent Slingshot:
Joyent Slingshot allows developers to deploy Rails applications that work both online and offline (with synchronization), and with drag/drop into and out of the application as in a standard desktop application.
There is also the Dojo Offline Toolkit, among many others.
Google has entered the offline app area with an opensource browser plugin called Google Gears that allows web applications to have an offline mode. Google Reader already has this mode. I’m guessing gmail will probably follow as well as the office apps.
Probably not useful for many library applications, but something to be aware of.