Of course there is no need to limit medical apps to the iPhone, Android and Windows 8 are or will be quite suitable.
The FDA is concerned about the robustness of the personal mobile platform. The OS's are not designed for mission critical applications. However, there are many applications where a re-boot would be acceptable. Also, some of the mobile processors are quite capable of running a secure OS.
Applications like real time heart monitoring could be run on present platforms.