From Amazon Customer Service (if you haven't done this already).
"If your Kindle is lost or stolen and you have a gift card/certificate balance and purchases are made, you will be out that money.
If you deregister the Kindle, it can not access the Whispernet (the Kindle's internet access), and it can not be used for purchases.
If you do not deregister it, the finder can deregister it directly from the Kindle, so you do not protect yourself by leaving it registered.
EDIT: Maggie Leung, a Kindle forum member, also pointed out that a thief could use a stolen Kindle to get into your e-mail accounts or other websites if your password is stored. This could let them get to confidential information. Deregistering will prevent this."
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