Apparently it is old news and was hot in the late nineties. I haven’t seen it before, and having worked as a sound designer I find this fascinating.
Aphex Twin has hidden an image of himself in the audio waveform of track 2 on Windowlicker. You need some software to find it and display it.
That’s what I call a cool easter egg on an audio CD. You know, like the easter eggs found in software, on DVDs and in movies. If you want to try this on your own productions here is a windows software that can help you out. Or you can use the Mac software that Apex Twin used to create the image above.
I haven’t found any cool examples of people doing the opposite, hiding audio in an image, but if you want to start implementing all kinds of data into other files you could try one of the following: Hide and Encrypt or Hider 1.1 (Both of them for Windows)
eirikso 
For those not too familiar with the fabulous Mr James here’s the understated wikipedia entry http://en.wikipedia.org/wiki/Aphex_twin
You might also want to listen a great peice by Nate Harrison on the Amen break (you definitely have heard this, unless you’ve been living in a sound proof cave for years)
http://nkhstudio.com/pages/amen_mp4.html
It’s also a real nice example of bringing a great story alive in a simple yet compelling way.
Pearse