Lots of Apple's Mac growth over the years has been people switching from Windows. I wonder what percent of people switch from Windows to Mac, and also what percent switch from Mac to Windows. I'd guess it is a much lower percent leaving Mac for Windows.
I'm a Windows -> Mac switcher, but even if I have to go back to Windows in the future, I'm sure I'll buy a Mac and run Windows on it. Every PC, especially laptop, I've ever had feels like giant plastic wiggly crap compared to the sleek little Air that's 2+ years old now.
Like many things, once you try a higher quality, it's harder to go back. After drinking an espresso, who wants to go back to instant coffee?
Sounds like exactly the same thing will happen with the iPhone.
It might be different if the price was vastly different. But all the top end phones cost about the same.