Originally Posted by Acreo Aeneas
I haven't seen any ebook reader device have that feature (unlike smartphones or PMPs). Would be a nice feature have in the event, you lose yours and you have hundreds of paid for titles on there as well as your account logged in. I'd be more afraid someone purchasing something with the stolen Nook.
The Sony 600 and 900 have device lock option (pin). In addition, when you access the online store, you're prompted for your password (username is always there). There's no way to purchase content without the login password. This is something that is always requested on the Amazon.com Kindle forum.