WhatIs.com explained about brain fingerprinting today. To summarise:

"Brain fingerprinting is a controversial technique that is advocated as a way to identify a terrorist or other dangerous person by measuring the "brainprint" of that person when shown a particular body of writing or an image that was previously familiar (such as of a training camp or manual). The brainprint is based on the P300 complex, a series of well-known brainwave components that can be measured. The technique is said to be more effective than a lie detector test."

What struck me first was the oxymoron name of this technique. I can just picture fingers sprouting from brains like plants. Not a pleasant image eh?