One of the most entertaining and clever posters I saw at WWW2005 was Enhancing the Privacy of Web-based Communication [pdf]. This is a new technique developed at PARC called "stealth encryption" and is basically aimed at preventing web mail providers – such as Gmail – from profiling you via your email. Words of a particular grammatical category are replaced with words from the same category and with roughly the same frequency in English. An example from the paper:
Plaintext: Hi friend, Its been a long time since I last wrote. How are your kids doing? I am immensely enjoying my stay here at PARC and California is even more beautiful than I had imagined!
Ciphertext: Hi young, Its been a support power since I last flew. How are your prices doing? I am immensely occupying my blood here at IBM and Kansas is even more environmental than I had illustrated!
I love this idea. It takes me back to all those dodgy WWII spy films, and some guy in an overcoat muttering "The sparrows fly over the clock tower at midnight".