Not 100%, but rather 90% refundable.
Link: http://store.apple.com/Catalog/US/Images/salespolicies.html
[search for restocking fee]
[search for restocking fee]
If you can prove its defective, there is no restocking fee, alternatively, they'll likely replace the phone with an new one if it is just a problem with certain models.
Apple claims this is the best that they have ever had and have explicitly advised their support staff that this is not a problem. Their recommendation is to just hold differently. In other words, its the user who is defective, not the phone.
Just how do you propose anyone proves its defective given that, and avoid being charged the restocking fee?
In general, Apple's retail stores are extremely straight forward, offer great customer service, and don't yank you around like other brick and mortar retailers.