Back in December of 2008, I wrote about browsers replacing OSes. Google introduced the Chrome OS yesterday, "an open source, lightweight operating system that will initially be targeted at netbooks".
The OS targets speed, simplicity, and security. "We're designing the OS to be fast and lightweight, to start up and get you onto the web in a few seconds". It will run on x86 and ARM, and will run on a new windowing system on top of the Linux kernel.
In addition, Chrome OS is separate from Android; while Android was designed to run on mobile phones and set top boxes, Chrome OS was designed "for people who spend most of their time on the web, and is being designed to power computers ranging from small netbooks to full-size desktop systems".