I've been thinking about this topic a lot recently and the almost apoplectic fits we've been seeing about the Chromebook Pixel's pricing provide and opportunity to respond to a common misunderstanding about Chromium OS and Chromebooks: They're not (just) browsers.
Of Interfaces and Widgets
One of my co-workers good-naturedly refers to my Samsung Chromebook as my "browser". I know he sees more deeply than that but it strikes me odd how many people really do miss a critically-important point about Chromium OS: Chrome the web browser is [[http://en.wikipedia.org/wiki/Qt_(framework)][a cross-platform application framework that is widely used for developing application software with a graphical user interface]].
More than a Web Browser
If you can get past the idea of a computer "that only runs a web browser" you can begin to see Chromium as a computing platform.
Not So New
What Google are doing with Chromium is the next logical extension. Look at Mozilla with XUL, Microsoft with XAML, Glade and JavaFX. They're all variations on a theme: describing interfaces in mark-up, separating data from display characteristics. Mozilla's XUL is the most similar to Google's strategy and it's been largely successful.
Remember the early days of the internet, back when Google was getting started and Netscape was coming down from it's ascendancy? This is where we thought the browser would take us. It's certainly where Microsoft feared it would take us. Why do you think they worked so hard to integrate IE deeply into the system?
If the browser becomes an application framework the OS becomes irrelevant. Microsoft had to attack the both the platform and it's number one champion. By integrating IE deeply they turned it into an application but not a platform. At least they retarded it's growth for quite a while. They didn't expect those search guys at Google to bring it up again.
Why Does It Require an OS?
If the OS is irrelevant in world where the browser is an application platform, why build Chromium. I think it's for the same reason Google had to build a competitive phone OS: to keep the dominant players from capturing the market and locking Google out.
Apple and the App Store
Apple have tremendous success with the iOS app store and are successfully transitioning the model to Mac OS. Apple have also been successful on iOS with email, calendaring, book and music sales, advertising and are moving into search with Siri. If Apple capture their own user base with their apps and services where does that leave Google?
The problem is even more magnified if Microsoft are successful in their attempt.
It Started with a Browser
While we'll never know for certain how it would have played out, I suspect Google would never have built Chrome or Chromium if iOS and the App Store hadn't shown Google a future that didn't include them. Once the path to lock-in became apparent, Google had to get in the game and provide viable alternatives.
What's interesting is despite all their competitors attempts at lock-in, Google still push open standards and open licenses. Android and Chromium are both open source. That's not to say Google are altruistic and don't want users searching at google.com. They do want those users. But they seem content to live in a world without lock-in, even their own.
 This realization leads to the conclusion that Chrome the web browser is a ported version of Chromium OS, much like Cygwin is a ported version of the Gnu userland to Windows.
 Anyone want to compare Qt, Gtk2 or MFC?
 Facebook provided another example of a Google-less future.
 Time will tell whether Google will stay the course. Will Google services move towards locking users in to the Google eco-system? Or will they make the eco-system so compelling no one cares to look elsewhere?