There’s no atomic instruction on ARM9 (I doubt iphone is using ARM6 - or 7).
Under linux, the atomic code is implemented by calling a kernel function pointer from userspace (to avoid process switch cost), see here: http://gcc.gnu.org/wiki/Atomic
As Iphone doesn’t run linux, either Apple implemented something equivalent, either you can’t use Atomic.
You have to know that ARM9 can’t load a 32bits variable in one shot (no 32 bits read is atomic on ARM9).
So the juce::Atomic class isn’t enough on ARM9 for real atomic operation anyway.