I'm not so sure about this. Customers and developers have significant investments in the existing app markets. Unless the mobile web catches up soon in terms of capabilities I think newcomers are going to have an increasingly hard time challenging the two incumbents.
Today users have no allegiance to Android at all. I know a lot of people with Android phones and no one, not one, is like, "I'm sticking with Android for my next phone too." They're all, "Maybe Android. But probably iPhone or maybe WP". iPhone users though are pretty diehard. In part because its still just the best phone, IMO.
But the lack Android allegiance is giving WP a window, no pun intended. Unless Android has some tricks up its sleeve for Ice Cream Sandwich, I think that WP can make serious inroads. Not 20% market share in a year, but climb back up to 10-12%.
I'm not seeing an iPhone devotion among many people that I would have expected.