It's supposed to commoditize Windows (or any other OS), making it irrelevant. The idea is that the user only ever sees the UI of the Browser, and forgets what Windows even looks like, so that you can swap it against any other OS without them even noticing. It's exactly what Netscape Navigator was supposed to become, a possibility which sparked Bill Gates' desire to "cut off their air supply". Lots of brawling ensued.
Correction: The "cut off their air supply" quote was attributed to MS executive Paul Maritz, not to Bill Gates, by Steve McGeady from Intel. Maritz later denied having said it <http://www.wpi.edu/News/TechNews/990126/microsoft.html >
There are Linux and Mac versions coming - they're just releasing first on Windows.