You may be confused about the difference between Android and Chrome OS. Here it is.
Chrome OS is Google's open-source operating system designed specifically for notebook computers. The search giant recognizes that for traditional PCs, using a mobile operating system just won't cut it. A laptop running on Android wouldn't make a lot of sense, so Chrome OS is basically a web-centric operating system designed to power more data-intensive functions and applications.
Meanwhile, the tablet market is something else entirely. Unlike some pundits, Google sees the two as completely separate worlds with a defined line between them. There is no blurring of the tablet/PC market for Google.
Engadget quoted Google's Senior VP of Chrome Sunday Pichai during his Google I/O presentation about the difference between Chrome OS and Android as saying, "There are a variety of experiences out there, and the web model is very different. We're comfortable seeing them coexist. Google Movies and YouTube have web versions -- when you use a Chromebook, you see how it's different, and they'll naturally coexist. These are very different models -- if we didn't do something like Chromebooks, I'm pretty sure someone else would."
So the addition of Chrome OS will not affect the marketplace as it exists now. It is not a change to the mobile environment - it is an addition to the PC software environment. If anything, that's the message Google wants to drive home.
But then again, since it is all open-source, what's to really prevent someone from running Chrome OS on a tablet? Maybe the lines will become blurred after all...