This sounds like a cool idea, but I don't think it would work in practice. The would-be thief, assuming he knows about the "duress mode" (which isn't a bold as

This sounds like a cool idea, but I don't think it would work in practice. The would-be thief, assuming he knows about the "duress mode" (which isn't a bold assumption considering the large black market for stolen iPhones), would recognize that you've logged in to something strange, that doesn't show any useful data. They'd just pull back the hammer on their pistol and tell you to try again.