Actually, yes!

In the early 70s I trained two cats, a male and a female, to use the toilet.
Here's how it was done......

a) place a regular litter box on the floor right next to the toilet. Leave it
there for a few weeks.

b) place a small plastic pan with litter in it so that it is held in place by
the rim of the toilet when the seat is up. Initially fill it with some dirty
litter from the box on the floor. Remove the box. Let the cats use this
litter pan for a few weeks.

c) cut out a hole in the bottom of the pan so that the poop and pee will fall
into the water. Surprisingly the cats will still use the pan. Let the cats
use the bottomless pan for a few weeks.

d) lower the seat (not the lid) so that the cats will learn to sit on the
seat while pooping and peeing in the pan.

c) remove the pan. Voila, the cats will now be using the toilet.

I didn't bother trying to teach the cats to flush the toilet!

Some owners have had success in getting a cat to use a toilet.   That
having been said the problem is getting the cat to flush.  And do
you want the cat to flush the toilet?

A couple of years ago Paul Harvey had a story about a guy who was
complaining about how high his water bill was.  He stopped complaining
when he was home sick one day and noticed his cat flushing the
toilet repeatedly.  

Oh and not to mention you need more than one toilet in your hosue.

And then there is the issue of do you really want to chance the cat
clogging the plumbing?  

Personally i'd stick with the old fashioned litter box and avoid
giving the plumber his sumemr home.

Yes....You can teach a cat to use the john, but I've never known anyone who
could teach them to flush afterward....Our cats can pull the sliding screen
door open to get out, but we have never been able to teach them to close
after they go out to keep the bugs from coming in......My father used to
say, "You can lead a horse to water, but if you can get him to swim on his
back, you've really got something...."