Back when I wrote about how a WebOS might work (basically XHTML/JS web apps that run on the desktop as well), I got a lot of responses along the lines of: with internet access becoming more ubiquitous (broadband, wifi, wireless broadband, WiMax, etc.), there will be less and less need for applications that don't need a connection to the network to function. When you can literally get a fast, cheap internet connection anywhere, you don't need a version of Gmail that works offline and so that's not going to drive the development of this WebOS thing you're talking about.
I've been thinking for several weeks about why I think that's wrong and I've come up with a couple ideas.
1. Fast, cheap internet everywhere? Hoo boy, wake me when that happens...you'll likely find me driving my hydrogen-powered hovercar with ESP to my paperless office.
2. For many people, the more you get used to having access to your applications/data/etc., the more important that access becomes. Let's say 98% of the applications you use are entirely on the web (with no offline capabilities) and you're online almost all the time wherever you go. Then the network winks out for 1/2 an hour. Or Salesforce.com is down for a couple hours. That last little inch is going to be painful. And no use telling me that sounds insane because I've seen the madness and fear in people's eyes while they clutch their Crackberries, furiously reading email mere minutes away from the office and the full-speed, full-screen experience.
3. The offline thing is a good way for companies to bootstrap the WebOS. I think most people have a sense that the apps they use in their browser are more alive, more social, more connected, even if they can't articulate that feeling. And whether it's true or not (Gmail isn't actually more "connected" than Outlook), companies can market the "aliveness" of their web apps (even when they run offline) versus the "deadness" of desktop apps.