The iPhone has the most to offer, in the easiest possible package - both for consumers and developers.
This is a very good point. The ability to integrate iPhone with the rest of your "digital life" is very easy. Read your news and check your stocks in the morning, throw it on shuffle when you jog, watch movies on the plane, control your stereo system, sync with MobileMe etc.
A big portion of iPhone is it's media friendliness. I'm not sure that it would be as popular as it is if it were just a mediocre phone with just an "ok" app store.
It's possible for Google to gain some momentum, just not likely in the first few months of the showdown, and still questionable long term. Let's not forget that many were skeptical about iPhone in the beginning too...
Apple's success will depend on whether they can deliver and innovate consistently though (and they have been able to nonstop), that's been the growing trend for the past few years. Otherwise they will lose it (once again, not likely). Not to mention, an iSlate/iTablet can also improve iPhone sales too... If the new tablet is the new hot product, you may see people who cannot afford the tablet settle for the iPhone...only to later buy the tablet and a Mac.