Android is free for device makers to use on their devices -- Google doesn't make a cent on the operating system itself. It's an interesting model that hasn't yet translated into money, but it has certainly translated into a ton of devices and end users.
Google is activating 160,000 Android devices per day -- whoa! That's an enormous number (that is accelerating). Imagine Google with a billion end users of their Android operating system -- something that can't be far from 5 years out given the current growth and acceleration. Eric Schmidt concludes that with that many users, all you'd have to do is make $10 per user per year -- and that isn't out of reach.
With that kind of user base, $10 billion per year isn't a stretch, given AdMob advertisements and app sales.
Windows Mobile 7 has potential too though. Even though it's not free, I can see the device being popular for end users based on some of the hands-on reviews you can find online. The operating system looks really great, and even though it's going to cost device makers money to use, I can see it being a popular choice for consumers.