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I could swear I have seen some blizzard links to the client that allowed to download it with out the need to pre purchase the game.
Found them
PC Version
enUS: http://eu.media.battle.net.edgesuite.net/downloads/d3-installers/589acdb4-c660-318a-b993-01528fad7da6/Diablo-III-8370-enUS-Installer-downloader.exe
EU-English: http://eu.media.battle.net.edgesuite.net/downloads/d3-installers/589acdb4-c660-318a-b993-01528fad7da6/Diablo-III-8370-enGB-Installer-downloader.exe
EU- Italian: http://eu.media.battle.net.edgesuite.net/downloads/d3-installers/589acdb4-c660-318a-b993-01528fad7da6/Diablo-III-8370-itIT-Installer-downloader.exe
EU - Spanish: http://eu.media.battle.net.edgesuite.net/downloads/d3-installers/589acdb4-c660-318a-b993-01528fad7da6/Diablo-III-8370-esES-Installer-downloader.exe
EU - French: http://eu.media.battle.net.edgesuite.net/downloads/d3-installers/589acdb4-c660-318a-b993-01528fad7da6/Diablo-III-8370-frFR-Installer-downloader.exe
EU - German: http://eu.media.battle.net.edgesuite.net/downloads/d3-installers/589acdb4-c660-318a-b993-01528fad7da6/Diablo-III-8370-deDE-Installer-downloader.exe
EU - Polish: http://eu.media.battle.net.edgesuite.net/downloads/d3-installers/589acdb4-c660-318a-b993-01528fad7da6/Diablo-III-8370-plPL-Installer-downloader.exe
Latin - Port.: http://eu.media.battle.net.edgesuite.net/downloads/d3-installers/589acdb4-c660-318a-b993-01528fad7da6/Diablo-III-8370-ptBR-Installer-downloader.exe
Lastin - Spanish: http://eu.media.battle.net.edgesuite.net/downloads/d3-installers/589acdb4-c660-318a-b993-01528fad7da6/Diablo-III-8370-esMX-Installer-downloader.exe
Mac
enUS: http://us.media.battle.net.edgesuite.net/downloads/d3-installers/589acdb4-c660-318a-b993-01528fad7da6/Diablo-III-8370-enUS-Installer-downloader.app.zip
enEU: http://eu.media.battle.net.edgesuite.net/downloads/d3-installers/589acdb4-c660-318a-b993-01528fad7da6/Diablo-III-8370-enGB-Installer-downloader.app.zip