I used to make my Gentoo installs as close as I could get to grayscale, for everything except content. Looked awesome, but like all other fiddly customizations, I basically gave it up after a while. Too much effort for too little reward.

[EDIT] Uh, the connection being that I would also collect grayscale icon sets, since sometimes the best results came from mix-and-matching those, as some might lack good icons (or any at all) for some programs.