Blizzard had posted this post which mentions that the initial installers had a bug. The post mentions:
Obtain the new Diablo III digital downloader using the links below
Point the download save at the same directory as before. The folder in question is your main game data folder. If you have the game
D:\Games\Diablo-III-8370-enGB-Installer, please point
the download to
Do not use the automatically generated folder containing the
installation files (i.e. do not use the directory
D:\Games\Diablo-III-8370-enGB-Installer as this is the one created by
If you chose the correct directory only 50MB will need to be downloaded to fix your install, otherwise you'll re-download the
Also, it's absolutely imperative that you download the same language & region client that you used to download the game after pre-ordering.
In my case, I had downloaded the En-GB version of installer & game after pre-ordering. However, today while downloading the new installer, Blizzard offered me the US version of the installer & because of this mismatch, it was downloading the whole game again.
After grabbing the En-GB(aka EU) install client, it was able to detect my previous download.