I read somewhere that Qt5, unlike Qt4, doesn't build to multiple platforms in one binary (in this case x86/arm, not sure whether it applies to OSes too), and then it clicked that the only binary for Mac builds to Mac x86 so I need to make my own for the iPhone (even the simulator). This page, specifically "Getting started" at the bottom, walks you through that process:
http://blog.qt.digia.com/blog/2013/03/05/qt-for-ios-preview/