The main reason why Android is winning is because they were the first quality (iOS like) multi-manufacturer OS that was pushed by a big company that most people love and got a lot of momentum. I don't think being free or open-source was the main reason Android is succeed. It definitely plays a huge role, but even if Google charged a $10 license like Microsoft is doing now with WP7, Android would've still succeeded.

In this case it's actually WP7 that is more like Linux - opportunity wise - because WP7 is the one coming into the market after Android is well entrenched.