Blizzard will soon send out Closed BETA invites to players who have opted in via their Battle.net accounts as well as give away keys through promotions and giveaways.

Yes folks, start checking your email regularly as the Diablo III beta test is officially underway. Of course this is only true if you have an existing Battle.Net account tied to any Blizzard game. If not, simply wait for any of the promotions and or giveaways that the Diablo Team will be doing at the Diablo III community site.

If you have opted-in for the the Diablo III beta and want to find out if you have already been selected for the Diablo 3 BETA, log into your Battle.net account, and click on the “ACCOUNT”. If you see the Diablo 3 BETA under your game accounts, you’re golden.

source Diablo III community blog