SpamMimic is a pretty interesting application of steganography, but once I thought of the matter a bit more, I found one big hole from the principle:
Suppose you live in a country in which encryption (and steganography, while they're at it) is illegal. Do you want to get busted for a) using encryption, or b) sending spam? =)
Lesson, however, has been learned - Like Macros the Black said in Feist's Magician, "Things are not what they seem to be." Hidden messages are everywhere. (Or maybe you're just paranoid.)
(Some conspiracy stuff follows...)
Of course, maybe the spammers have been using a method like this all the time! This may be the secret channel through which they pass all the stuff they don't want the honest netizens to know of, like the list of dangerous individuals and stuff...
...or maybe not =)
Then again, if you have wondered what the random numbers and letters in Usenet spam headers and end of the messages really mean...