Darkmail Attacks - The Next Network Threat?

An anonymous reader wonders: "SC Magazine are running an article on the growth of so called Dark Mail Attacks. Whitedust Security appear to have identified this as a potential problem way back in December 2004. Since that time, a marked increase in attacks of this nature, including the recent attacks on the UK Government infrastructure, have been recorded. Are these types of attack a new large scale threat or just a passing fad?"