Initial invitations to join the beta testing program for Blizzard Entertainment’s Diablo 3 game are finally making the rounds to account holders.

The start of a beta for an online game usually indicates that the game will come out relatively soon. Diablo 3, one of the holiday gaming season’s most anticipated titles, was originally slated for release later this year. But the company is still not committing to a 2011 release date for the game, said Blizzard Entertainment chief executive Mike Morhaime in the company’s most recent conference call discussing its second quarter earnings.

The Diablo 3 beta includes the first “act” of the game, one of several large chunks of the game, that consists of part of the game’s main storyline. All five classes are playable in the beta. The beta of the game is around 3 gigabytes in size, which is considerably smaller than Blizzard Entertainment’s other online game, World of Warcraft. That game is around 26.6 gigabytes, including all recent patches and content.

The last Diablo game came out in 2000 and still boasts a large player base. It’s nothing compared to Blizzard Entertainment’s most popular online role-playing game, World of Warcraft, which has around 11.4 million subscribers. But Diablo’s hack-and-slash and loot-seeking formula proved to be popular for more than a decade. It even inspired a number of dungeon crawler copycats like Torchlight.

The Diablo games allow anywhere from two to eight players to jump into discrete games in randomly generated dungeons. The goal is to prevent the forces of hell from taking over the world by defeating Diablo, the lord of terror, and his cohorts. Players chase after powerful items and weapons to use against other enemies and players or for trade.