Indeed a good idea for producing multi-platform applications is the web. That's the web as we have it. If an app needs to interact with the operating system (e.g. a cd-recoding app) then the web of course doesn't work (there will be a <device> tag in html5 for microphone and video support afaik, I wonder is that something that's supposed to expand, but I still dont see it). If that's the case you need a native app and Mozilla makes XULRunner which is a packaged gecko so that developers can make native applications using the gecko-supported technologies (xul, html, css, etc.. but thats just the frontend stuff and not most of the purpose an app would chose xulrunner). Examples of xulrunner applications are firefox, thunderbird, and songbird, and there are more..
A webbrowser indeed needs to interact with the operating system. Or at least the browser backend needs to. But the frontend. If as firefox is developed with complicated xul that is only supported by gecko, we built frontends using html, css: standard web technologies, then we can build a highly extensible system where it's possible to effectively switch between front ends (and without a restart, perhaps in an instant on a fast computer).
So picture instant browser themes that chrome/firefox 3.6 have: only the whole interface is customized.
For the record, I think chrome/ff can do better by supporting semi-translucent and live scripted backgrounds. (ff does support animated pngs, but so much more should be possible)
Additionally if the multi browser thing works we have the effectiveness of your fully customized browser and web info in the cloud that you can use on your mobile, at work and at home without hassle. (Now, Mozilla has the Weave extension to achieve this for Firefox, and Opera has it's own solution I'm sure. Chrome would be wise if it hooks up with Google Notepad and an eventually bigger service to save more than just urls but session tabs etc (does that sound scary to anyone else?). But the point is this web integration comes more naturally for web applications. Particularly if your web browser is a webpage: then you can use it anywhere.)