Web OS? Not an OS
tbray.org
tbray.org
"OS"es started as collections of low-level utility code that helped apps run on particular pieces of hardware. Code that loaded programs, provided low-level I/O services (Bray bags on the primitiveness of OS services, but they're much better than talking to h/w devices directly), helped with memory management, and so on.
The web doesn't really have hardware, but it definitely needs abstractions and utility code. So I'd say the "Web OS" is something like Mochikit, or Dojo, or Prototype, or any other big honking JS framework that abstracts away client-side nastiness.
At any rate, that seems the part of the "Web" that's most analogous to a traditional OS, and something usefully modeled as such.
s/low-level utility code/APIs/
s/particular pieces of hardware/individual websites/