Officially fake. Check out the "making of" video linked at the below Snopes article.

http://www.snopes.com/photos/advertisements/megawoosh.asp

The biggest clue that this is fake...He's going head first when he starts down the slide, but when he is launched off the ramp, he is suddenly feet first.

Great little stunt. It's fake but looks great.

here's how it was done: Guy at the top of the slide rolls off the slide at the low part, which is obscured by the camera angle and the hill. CG is used to show the "guy" flying off the slide, through the air and splashing down. A second actor was in the tub, submerged while the camera was tracking the area where the CG was to be comped in. When the camera pans to the tub, he emerges, as though it was the same fella "flying" through the air.

It could be real... if it ended with paramedics rushing the tub and not three douchebags in North Face coats. Watch any real stunt, and you'll see that the "first responders" are always the guys with the medicine and fire extinguishers.

Also, his back did not appear to be broken, which it most definitely would be.