I'm an Android developer, so I'm probably biased, but I think most people in the developed world will have a smart phone eventually, just as most people already have access to a PC and Internet connectivity.
I think the ratio of phone / PC use will vary greatly depending upon the person's lifestyle. If you're a city-dwelling 20-something student you're going to be using your mobile phone a lot more than a 60-something suburban grandpa.
This isn't because the grandpa's old fashioned, it's because the two people live in different environments and have different patterns of work and play.Will people stop using PCs? Of course not. At least, not most people. There are huge advantages to having a large screen and a decent keyboard and mouse. But I think people will start to think of their phone and their PC as two views on the same thing -- the Internet. And that will shape what apps they use on both the phone and the PC.
And this switching will be a strong force towards having people move their data into the Internet cloud, so that they can access their data from whatever device they're using. This tendency will be strongest with small-sized data that originates in the cloud (like email), but will probably extend to other forms of data over time.