I think it all depends on where you live too. Some carriers have better coverage than others.
I live in SoCal, and iPhones is no longer a fad, it's a norm. At some point, when everybody has one, the marketshare will shrink. Think of the Nintendo Wii. It sold like crazy, but now that everybody has one, Xbox 360's are outselling it now. Once you reach the top, eventually there is a decline. Nobody can stay on top forever. Cell phones is similar to pop artists in music where you can become yesterday's favorite in a heart beat. Even the Motorola RAZR had a decent run before flip phones became obsolete because of touchscreens.
Of course, not saying iPhone sales are shrinking. The year of 2010 has been a banner year for Apple. But eventually, this industry deals with trends and iPhone no longer is the fresh face in the game. People do want change and variety. I guess it is like buying a new car. After your lease is up, do you really want the same manufacturer and color again? Some of the new Android users are expatriates of BB or iPhone. But going back to my first point, it could all depend on location. Then from that point, maybe people already got their "iPhone fix" during the last three years and want to try something new in their next contract. Perhaps staying locked into an iPhone contract for a minimum of 4 years is too much. An iPod touch/iPad helps resolve any iOS withdrawals.