Just so everyone knows, the D3 installer is already available to download, so I suggest you start downloading the 7.6GB file now.
PC:
http://us.media.battle.net.edgesuite...downloader.exe
Mac:
http://us.media.battle.net.edgesuite...loader.app.zip
Download the above small file, run it. It will download an encrypted client to a folder you select. On May 14th, run the setup.exe in the folder where you downloaded said files; it will install the game. May 15th, run the installed game, unlock it with a code from the box (or however digital downloads will unlock), and play!