It's faked in fact it was part of a viral marketing campaign for Microsoft Office 2007 in Germany, but it took off much faster than anticipated, undermining the actual product connection.
There's some CGI involved (the body in the air) in addition to piecing together a selection of clips in a nearly seamless edit. Although it looks real, there are some telltale markers.
The composition of the video uses an intentionally shaky, handheld look and feel to hide the seams between the edits, a trick that has gained a lot of popularity in viral videos.
The biggest giveaway though is the physical impossibility factor. The slider gains far too much speed far too fast, but it looks somewhat plausable due to the perspective of distance. The biggest implausability involves the variations possible in an uncontrolled slide over that kind of distance. Under those circumstances, any slight variation would prevent the kind of accuracy displayed. An inch or two to the left or right at the launch point or a slight variation in speed or wind resistance would result in a dead slider. However, don't take my word for it, you can find out exactly how it was done and see some unedited footage as well.
http://newteevee.com/2009/08/11/the-megawoosh-waterslide-viral-how-it-was-really-done/
Hanley