I've been playing around with a handful of WebOS's as of late. In the last six months they have sprung up like hot cakes. These web-gadgets are wonderfully cool and wonderfully useless. The value of having an internet desktop on top of your own desktop seems slight. I could only see it being useful for someone paranoid about privacy or who jumps around between dozens of computers. But even then, there are better tools for doing either job. And if you're paranoid about privacy you're hardly going to put all of your trust in some quirky WebOS company. Especially given their history.
I also don't understand why nearly all WebOS designers decide to implement their "OS" in their own proprietary AJAX/flash system. Every WebOS has to write their own text editor, calculator, web browser, file management system, etc. It seems very inefficient to rewrite all of those programs. Not to mention that the quality of the apps are much much worse then their desktop counterparts.
Instead of writing a whole new operating system with a closed set of apps, it makes much more sense to provide a web-interface to a real computer or virtual machine. The reason why Windows, Mac, and Linux are popular is because its easy to write software for them. If any WebOS ever takes off they are going to have to be able to run lots of software.
One implementation may be a WebOS sever which simply finds a user's virtual machine image and loads it up. It then launches a browser plugin like VNC or remote desktop. You then get access to a whole computer, not some mushy half-ass web-app.
All of these pieces already exist. All you need to do is hook them up. You could use the open source implementation of remote desktop, the open source virtual machine software, and then either find or write some kind of user and security management front-end. I'm kinda amazed no one has done it yet and called it a WebOS.