Diii.net reports certain sites are claiming to have a Diablo III demo leak. If you see a site on the search engines in the net or on a torrent that claims to have a Diablo III demo be very wary of downloading it. There is no leak of this game. Leaks usually happen when a friends and family tester or beta tester breaks an NDA. Diablo III is not in Alpha nor Beta testing. It can only be played on Battle.net 2.0.
Use common sense. What is being distributed is a trojan horse / malware. Beware.