I think the closest you can get to this idea is AOL where they store all your data on your server and give you a good selection of web apps for calander,browsing,etc. They could possibly change the way it works so it looks more like a desktop (aka the old netscape vision) but I dont think this could be called an OS.

Even for a cell phone, the idea that I wouldnt be able to even scan my address book or emails without being able to connect to the internet doesnt seem like a good idea unless connectivity to the web is 100% guaranteed and at LAN speeds.

I think you are leading to a vision where all your data and programs would be on some remote server and you can go over to your friends house and have your programs/data available to you and also be able to have them backed up so if your terminal dies, you just have to buy a new one without reinstalling everything. This could be delivered through a VMWare style system where things are installed to an encrypted VMWare session that is actually installed and run locally but the VMWare session is actually mirrored on the net somewhere. if you go to a remote desktop, you could either try to run it like a remote desktop or if you want faster performance, you could download the VMWare session and run it natively on that remote terminal.