A business mans company tell him that he will have to take a long business trip to japan to clinch an important deal. Unfortunately his wife is known for cheating on him when he goes away. He loves her still but everytime she does it it breaks his heart and this trip will be the longest he has been on. He also knows that she tries hard not to cheat and has done less and less. Having given up smoking he knows how hard it is to shake an addiction so he decides to go to a sex shop to get her something to keep her amused.
On his way home he goes to a little sex shop. He looks around for a bit but doesnt really find anything satisfactory. Just as he is about to leave the owner calls him over.
"Your looking for something special?"
"Yes, i need something to keep my wife busy while im away so she wont cheat"
The owner looks at him hard then reaches under the counter and pulls out a small wooden box with mystical carvings and pictures on it. He slides off the lid and inside sits a carved wooden dildo.
"Whats so special about that?" asks the man
"Watch... Voodoo dildo door"
To the mans suprise the dildo rises from the box and starts fucking the keyhole of the door.
"Voodoo dildo box"
The dildo stops and drops back into its box.
"Thats amazing i'll take it"
After paying for it he walks home with a smile on his face. The next morning after packing he gives his wife her present. At first she is dubious but after showing her the door trick she seems quite pleased, so he leaves on his trip.
After a week she feels then need for a shag but as she wants to stop cheating she instead gets out the dildo.
"Voodoo dildo my pussy"
The dildo rises obediently from the box and starts to shag her brains out.
A hour and ten orgasms later she feels better. Through all the sexual exstasy however she forgets the turn off command. And as she is well into orgasm numer eleven she cant think straight. she trys to pull it out to stop it but it doesnt work, so she deciedes that she will have to go to hospital to have it removed.
In the car on the way there she has another orgasm that makes her swerve dangerously. A police officer see's this and pulls her over. He walks up to the window which she rolls down.
"Have you been drinking?"
"No" The now destrought woman replys
"A voodoo dildo is shagging me and i cant get it to stop im on my way to hospital to have it removed"
"Voodoo dildo" the officer laughs "My arse"

Here's the version I got about 10 years ago .... same gist but slightly better storytelling in my opinion. Enjoy!

There was this businessman who was getting ready to go on a
long business trip. He knew his wife was a flirtatious sort, so he
thought he'd try to get her something to keep her occupied
while he was gone because he didn't like the idea of her
screwing someone else. So he went to a store that sold sex toys
and started looking around.

He thought about a life-sized sex doll, but that was too close to
another man for him. He was browsing through the dildos,
looking for something special to please his wife and started
talking to the old man behind the counter. He explained the
situation.

The old man thinks and says, " Well, I don't really know of
anything that will do the trick. We have vibrating dildos, special
attachments, and so on, but I don't know of anything that will
keep her occupied for weeks...except--" he stopped.

"Except what?" the man asked. "Nothing, nothing at all."
"C'mon, tell me! I need something!"
"Well,Sir, I don't usually  talk about voodoo dick'."

"So, what's up with this voodoo dick?" he asked. The old man
reached under the counter, and pulled out an old wooden box
carved with strange symbols. He opened it, and there lay a
ordinary looking dildo. The businessman laughed, and said, " Big
fucking deal. It looks like every other dildo in this shop!"

The old man replied,"But you haven't seen what it'll do yet." He
pointed to a door and said, "Voodoo dick, the door." The voodoo
dick rose out of its box, darted over to the door, and started to bang
the hell out of the door and a crack developed down the middle.

Before the door could split, the old man said, "Voodoo dick, get
back in your box!" The Voodoo dick stopped, floated back to the
box and lay back in there.

"I'll take it!" said the businessman. The old man resisted, saying
it wasn't for sale, but he finally surrendered to $700 in cash. The
guy took it home to his wife, told her it was a special dildo and
that to use it, all she had to do was say, " Voodoo dick, my  ...."
He left for his trip, satisfied that things would be fine
while he was gone. After he'd been gone a few days, the wife
was unbearably horny. She thought of several people who would
willing to satisfy her, but then she remembered the voodoo dick.
She got it out, and said, "Voodoo dick, my pussy!"

The voodoo dick shot to her crotch and started pumping. It was
great, like nothing she'd experienced before. After three
orgasms, she decided she'd had enough and tried to pull it out,
but it was stuck in her, still thrusting. She tried and tried to get out and to stop.

You see, her husband had forgotten to tell her how to shut it off.
She put her clothes on, got in the car and started to drive to the
hospital, quivering with every thrust of the dildo. On the way,
another orgasm nearly made her swerve off the road, and this caused her
to be pulled over by a policeman.

He asked for her license, and then asked how much she'd had to
drink. Gasping and twitching, she explained that she hadn't been
drinking, but that a voodoo dick was stuck in her pussy,

and then he  said, "Yeah, right Lady. Voodoo dick, my ass!!!"