An interesting read about how any usb device could be a potential threat. Some scary extracts:

Once reprogrammed, benign devices can turn malicious in many ways, including:

• A device can emulate a keyboard and issue commands on behalf of the logged-in user, for example to exfiltrate files or install malware. Such malware, in turn, can infect the controller chips of other USB devices connected to the computer.
• The device can also spoof a network card and change the computer’s DNS setting to redirect traffic.
• A modified thumb drive or external hard disk can – when it detects that the computer is starting up – boot a small virus, which infects the computer’s operating system prior to boot.

And a scarier one…

No effective defenses from USB attacks are known.

Once infected, computers and their USB peripherals can never be trusted again.

Some proof of concept should be introduced in a week at the incoming Black Hat convention. This is gonna be good :)

EDIT:

There’s actually something similar that you can already buy: The USB Rubber Duck