http://www.citikitty.com/
On this site, you can buy a kit containing all the necessary materials needed to teach your cat how to use a toilet instead of a litter box. I am so tempted to teach my cat.
It is actually an intriguing system. First, you place a shallow pan (that comes with the kit) full of litter onto the toilet seat. Show your cat where it is and take away its usual litter box. The cat will use this pan. After a week or so, you replace this pan of litter with one that has a small hole in the middle. The cat will use this as it did the previous one. After another week or so, you replace it with another pan with a bigger hole in it and another afterwards. After several weeks, you will be able to remove the seat cover entirely and the cat will use the regular toilet seat like a human.
Teaching the cat to flush requires a bit more enginuity, but is not impossible.