In your Admin panel, go to: System > Configuration > Design
Under “Themes”, you have to complete the fields with the corresponding identifier of the theme you are setting up (eg.: HelloCanvas would be hellocanvas)
Under each field, click in “Add Excepion”, in “Matched Expression” you have to write: iPhone|iPod ,
in “Value” write: “identifier_iphone” (eg.: hellocanvas_iphone).
Magento automatically detects if the browser is opening the theme from an iPhone, iPod, or iTouch, and reads wich theme is from the “Value” option.-