This morning I was playing around with a soft-synth and didn't have a midi controller hooked-up -- so I was using the QUERTY keyboard to play notes. Then I realized I could type words and thereby make cute little melodies, that would repeat and had themes... pretty interesting sounds, actually. I liked the idea that these melodies could be decoded back into text (with a little effort).
A few googles later, bing!
Here is Hello World in C Major:
Read the whole article here: http://www.codeproject.com/KB/security/steganodotnet20.aspx