[alister]

How about authenticating with an accelerometer rather than through skin conductivity? To unlock your USB device, or to avoid the self-destruct, you would wave it, tap it, spin it, or rotate it in a specific way before plugging it in.

Accelerometers are widely used by smartphones so I assume that the ICs are common and inexpensive (at least if bought in volume). A tiny battery would be needed as well.

Carrying the idea even further, maybe you could even do key entry this way. Imagine that the USB device stores encrypted data and you enter a short decryption key by tapping out a particular bit of music before plugging it in.

To get even fancier, maybe you can enter a hexadecimal key (0123456789ABCDEF) through various motions. For example, spinning the USB clockwise on your desk is a 0, flipping it from horizontal to vertical is a 1, waving it through a figure eight loop is an 8, rolling it along the long axis is a B, etc.