It appears as if most of the increase in iPhone market share in the US comes at the expense of the Android platform.
While the Apple success on the US market may not be repeatable in every global market because of handset cost issues etc, I think it is entirely possible that we can see similar trends in other developed markets because of how smartphone trends seem to originate in the US market at the moment..
I also think that unless Google makes a move, they will lose further market share to Apple once users decide on which device to get as their second smartphone. There is a risk that some current Android users will trade up to the iPhone because of a multitude of reasons such as, App availability, premium design or just peer pressure.
If this happens, Google will be attacked from two directions, because as I recently said, they are vulnerable to attack from Nokia and Microsoft from the low end (when it comes to price/performance ratio).