This NY Times article gives a little idea of operating systems sales on mobile phones. From this article and other sources, it looks like Apple thinks they will sell around 10M iPhones next year (either next year, or in the first 12 months), and Microsoft is projecting sales of 20M Windows Mobile devices in 2008, both of which were announced before Google's Android announcement today. So the mobile operating system war is much closer than the desktop operating system battle.
Also interesting: With Eric Schmidt (CEO of Google) being on the board of directors for Apple, I wonder what the rules are about disclosure when a board member's firm (Google) is developing a competing product for the company you're advising (Apple)? Did he have to excuse himself from any board discussions of Apple's iPhone? Or did Apple already know about Google's plans?