Taking this as a description of the three rotation axes: https://en.wikipedia.org/wiki/Aircraft_principal_axes
Looking at an iPhone laying flat on it's back, screen facing to the ceiling, and imagine the top of the phone (the end where the camera is) as the 'nose'...
I would argue that it is currently not possible to detect Yaw - rotation of the phone while it lays on it's back without being lifted.
I am currently testing this with Matt Pringle's Corona Remote and using the various values returned to move a dot around the screen.
It doesn't matter what I do, there is no motion I can put the phone through while laying on it's back which will get the dot to move (an appreciable distance) - and this is while multiplying the values returned by 100.
My ultimate goal here is to use the phone like a laser pointer, but this is currently not looking like an option. Any ideas?