Note for digital copy people:

If you downloaded the pre-sale client, you should have a folder called "Diablo-III-8370-enUS-Installer" where ever you specified the downloader should put it. Inside that folder is a file called "Diablo III Setup.exe". You just need to double click that file. The installer unlocks on May 14 at 8:01 AM PDT so you'll have 3/4 of a day to get an install issues worked out before the game servers open 16 hours later.