Here is a very interesting company, Flatwire. They make and sell cables that are extremely flat/thin and can easily be hidden by paint.
They make data cables, video/audio and even electrical cables. What I find very nice about this is that its so thin it makes installation very easy, without the need of ripping through walls or ceilings.
The only problem i find with this, is the price. Its rather expensive for a simple ethernet, but heh, very cool.

Check it out: Flatwire