1) I don't have an iphone, but found this to be an issue (if i understood correctly) that will be resolved in ios6, but for now what is recommended is to change the code that Ismael provided to:
echo '<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1">';
2) You could also try the code found here, which has more to do with correcting the orientation bug in iPhone, and heavily touches upon the zoom functionality: Just add it to the end of /js/avia.js file :
3) On an unrelated issue, you are loading you Analytics tracker twice, one with custom tracking, and one without. You may want to check WooCommerce plugin and Yoast to make sure only one of the two is used for Google Analytics.