As some of you may already know, the Diablo III BETA client has been leaked as of four days ago (2 September 2011). It only recently went hot within the Battle.net dev. communities. (6 September 2011)
Please use Google as your reference. I will not post links here for copyright or other trademark reasons. As for other references, you can also visit DarkBlizz.org for a good amount of info (including a link to the installer), though you must navigate yourself to the respective posts.
As far as I know, the installer is released in Chinese and English (US), but there may be others floating around that I have not seen or searched for. Both installers were digitally signed by Blizzard Entertainment, Inc. on Friday, August 19, 2011, you can check for yourself if you download one; this is proof that they're legit.
Use all of this at your own discretion. I have not seen any videos or screenshots of anyone being past login; that is to say I have not seen anyone actually playing the game yet. All I've seen so far is people being at the login screen with changeable options such as video/sound and gameplay.
As always, have a wonderful time ;)