As you said, it has become commoditised. Is there really a qualitative distinction? Apple's operating supports Apple's hardware (Mac and iPhone). Linux claims great compatibility with most PC hardware and tonnes of other random equipment out there. Windows is Windows. Do you think the average person cares what file system underpins the iPhone or that Android and the Pre share an OS kernel?

But you are right, you can't replace an operating system with browser. However, you can treat the OS as just another layer in the stack in between the network card and the browser. It's even an interchangeable component -- swap out Linux or Windows or OS X and you have still have something that can push bits onto the net and run a browser.