kdarling's figure is actually from 2012. Samsung hit their high between 2012 - Q3 2013 and proceeded to lose some of the share in the late 2013. In November 2013, Apple reportedly had 65% of the $400+ market worldwide, compared to 35% a year before, while Samsung's share was reduced to 21% from 40% a year ago. (
Reference)
Regardless, defining the high-end market as "$400+" doesn't tell the whole picture because Apple concentrates most of their efforts in the $500+ devices compared to Samsung who has a lot more devices in the $400-500 segment.
The troubling sign for Samsung is that they've already used up the obvious drivers for growth, namely a large product portfolio, strong carrier distribution, and extravagant marketing spending, whereas Apple still has some unplayed cards lefts in those areas.
The interesting thing is how it seems nobody foresaw Samsung's sudden deflation in the market as now they have seen three consecutive quarters of decline YoY from them. By 2013 the Apple doomsday scenario was in full effect and yet it's Samsung who's seeing a bigger decline first, mostly for the exact reasons analysts said, but for Apple. Here's a good writeup by Ben Thompson at Stratechery about it:
link.
Of course, all this doesn't mean Apple won't run into the same problem faced by Samsung in the near future, and their move to introduce more integration within the OSX-iOS ecosystem is their attempt to fight that off by providing more reasons for users to get the iPhone over Android phones. The result remains to be seen.
As a side note, I thought Google's recent moves with Android were very much in the dis-favor of Samsung in a way since it was mostly eliminating differentiation and encouraging more race-to-the-bottom behaviors in the OEMs, which is mostly not a good news for Samsung.