This one is so simple it's shocking that Apple missed it in testing:
a Smart Cover can essentially unlock an iPad 2. The person who unlocks your iPad 2 will not have complete access to your iPad, but will be able to gain entrance to whatever you locked your iPad 2 on. If your iPad 2 went to sleep in Mail, Safari, Messages, Contacts, or Maps, you can imagine the sorts of personal information that can be viewed on your iPad. If you left your iPad 2 on its Home screen, the person can view which applications you have on your device, control media from the multitasking bar, but not much else.