Web based OSes are the stupidest ideas ever. Unless you trust your personal information with a server owned by a company. Sorry, I feel safer with my harddrive. If I want to access it on the go I'll get a laptop.
Things like Web-based OS' are already in massive use. I don't know if you've heard of Citrix, but it's pretty similar to this idea. A client runs on a local OS (something like Windows CE on thin clients, but any OS that has a supported client for the ICA protocol works) and connects to a Citrix server, which publishes a desktop of multiple applications.
For lower budget companies, this sort of thing is a life saver. My school has been using Citrix for a while.
I totally agree, though. I'd never use something like this on my home computer.