I saw something like this in an episode of Wired Science on PBS, but instead of using electrodes in the eye, they used a grid placed on the surface of the tongue, and relied on the brain to remap it to vision. From the comments by users, and the experiments they showed, it worked pretty well. I think they had a similar version for people who can't balance due to inner-ear problems.

Don't steganography tools already do this? Seems like you could just generate enough random bits, and set whatever software to replace all the however many least-significant bits with it.

There's more stuff like this at http://amasci.com/amateur/beads.html