John Gruber recently pointed out a pull-out tab on the HP Slate 500 which does nothing but display licensing and regulatory information. I agree, it’s quite silly. However, the Apple Xserve has something very similar.
On the back of the device, you’ll find a tab that slides out to display the machine’s serial number. That’s all it does.
Of course, an Xserve is not a consumer device, and you can retrieve the machine’s serial number without using the tab. But still, there it is.