Interesting that it hadn't been found earlier. I had been searching for almost two years for the demo. The Riven demo I found was on the SoftKey release of the Prince of Persia Collector's Edition. Seems Broderbund/Red Orb like to hide their demos fairly well (On a side note, reverse the first six letters of Broderbund for a neat trick :P).
Are you still looking for a Riven gameplay video (*coughStrangerkecough*)? Well.... there is none, because this is all part of