It will be very interesting to see what happens once smartphone ownership in the US saturates and any further sales come from people replacing smartphones, not dumb or feature phones.
Right now there is still some low hanging fruit as dumb and feature phone owners come in to see what's available for cheap or free when their contract renewal rolls around. Naturally the carriers are very keen on flogging their smartphones, and their are plenty of cheap or free Android handsets to get people to make the switch. I haven't been into a Verizon store recently but I wouldn't be surprised if dumb and feature phones are being treated like LPs after CDs had taken over-- a few dusty choices in the corner for Luddites.
All of which is just fine and dandy, but it does mean, as has been suggested, that some number of those Android phones are simply being used as replacement feature phones, so their users are unlikely to be particularly wed to the platform.
So I'm curious what the numbers look like when pretty much everyone has a smartphone, everyone who's interested in using such a device to its full capacity has learned how to do so, and it's no longer possible to wow anyone simply because a phone has a functional browser.
I'm not saying that there will be some kind of mass migration to iOS at that point, but I do think there may be at least some shifting of percentages as more sophisticated users pick and choose based on experience rather than just whatever the guy at the store was pushing.
Ever heard of the phrase, "Get 'em young."?
We could be seeing the same thing play out. There's a reason that Apple pushes the iPod Touch so much. It's a training device for an iPhone.
Likewise, those folks getting a cheap Android to replace their old dumbphone/featurephone are suddenly going to discover the joys of Google Navigation, integrated GMail and Google Search, etc. Once they do, they'll stick to a smartphone, and a good chunk of them will stick with Android. Some will move to other platforms, but the vast majority will stay with the platform they learned to use first, as long as it remains technologically relevant. Just look at how long Symbian and Windows Mobile lasted in the face of the iPhone and Android.