MIT Technology Review's 10 Emerging Technologies of 2008 report includes offline web applications as a rising trend. When developing Ajatus, our new P2P personal CRM the offline issue was often in our minds. We even wrote in the manifesto:
Ideas may come to you when you're sitting in a bus, boarding an airplane or visiting a hospital. For a CRM to work the data must be available and editable in any situation.
Having now dogfooded Ajatus for almost two months, I have to say this has been an important aspect. It is powerful to have all your customer and project data with you at all times, and still be able to use it via the familiar web UI.
Offline is useful. You can write your notes in every meeting, update them in train, report expenses as they incur. And most importantly, as long as your computer is running, the application is never down. And still, though replication your data will be safe with your peers or the corporate server.