You wrote:
"The bottom line is that I fully expect smartphones to become the most widespread global computing platform in the next five years, driven heavily by the developing world, where the smartphone will be the primary PC for the majority of users."

Either you don't live in a developing world (which of course is true) or you have not been to one.

I am from Asia and I am also an iPhone app developer. The following are some of the reasons why the gist of your article will not hit bottom in Asia, or as you say developing world.

1. Smartphones' exorbitant price. A US$300 and above price tag won't get mass appeal. Asians are text messaging (SMS) lovers so they may not need those other smartphones features. Only a fraction of the mobile users have smartphones.

2. 3G internet access' exorbitant cost. In my country the cost for an internet access using iPhone is very very high. So what do we do? We turn off 3G and change the provider's APN in iPhone. Many iPhone users in my country were surprised by the sudden increase in their mobile billing (including mine). Many iPhone apps (both by Apple and 3rd party apps) are notorious in accessing the internet through cellular network even when you're not doing anything (or maybe forgot you have 'clicked' or set something). So since my last exorbitant bill, I became a WIFI only iPhone user--that is, if there's a free one nearby.

Given above reasons, I wonder now how your prediction would become true in the developing world in the next five years? Your Google vs iPhone app is a non-issue at this time for most of us. Let them lower the price of the mobile hardware as well as the 3G fees first, then let's talk about your article again. So for now, leave us developing world with our low cost and sometimes free text messaging.