The Inquirer techie-rumour site has this post about spam being targeted as a national security threat. The obvious indirect threat (that spam overwhelms the eavesdroppers) is a prime consideration, it seems. But I think they miss a cleverer use: steganography.
One could use a standard spam format -- say a Nigerian letter -- with apparently random words attached. Most of the time the words ARE random, but on Der Tag the go order (e.g. "frumious bandersnatch") is embedded into them. Send these out to the typical jillion recipients, including your agents. The agents set their spam filters to trap the go order. This totally defeats any possible Echelon system, the recipients are not identifiable, and the sender looks like just another spammer.
So, let's treat this as the threat it is. Open up a new Guantanamo, say in the Antarctic, and start chucking spammers into it. Just another side benefit of the War on Terror.