From the comments at macosxhints.com comes an ingenious method for defeating hardware keystroke loggers:

The method above sounds adequate for techie types. I propose the following alternate method:

1. Buy a cat. This also necessitates cat food, litter box, etc., to meet said cat's needs. The cat must be installed at the same facility as the computer, and with access to the computer at will.

2. Spray the computer keyboard with catnip scent, and optionally decorate the keyboard and surrounding area with items likely to attract the kitty.

3. While away from your computer, switch to an application that can accept random input without harm to your system or files. Leave the keyboard out in the open.

While you are out, the kitty will play on your keyboard, filling up the memory buffer of any USB intercept device that may be present. Eventually the memory of the device will be full of complete nonsense keystrokes, thus rendering it useless to the perpetrator.