Blizzard has introduced a new program for would-be players of Diablo III called the Diablo III Starter Edition. The only way to obtain the Starter Edition is through a guest pass, which comes with every retail box copy of Diablo III.
The Starter Edition provides players with what amounts to demo access to the game when it comes out. In the Starter Edition, players can level their character up to 13 and fight their way to the Skeleton King in Act 1 of the game. Matchmaking is limited to other Starter Edition players, and players do not have access to the Real Money Auction House.
All characters created on the Starter Edition can be upgraded to retail characters once you buy the retail version of the game and upgrade your Battle.net account.
Players who don't have access to Diablo III at launch tomorrow can (presumably) install the Starter Edition and play with that while they wait for their copies to arrive.
Here's the Battle.net post explaining how this all works:
Diablo III Starter Edition provides players the ability to try out a limited version of Diablo III. Guest Pass keys allow a player to give friends the opportunity to play the Starter Edition of Diablo III. Guest Pass keys are included in all box copies of Diablo III.
Players on Starter Edition game licenses have the following restrictions:
– Act I up to the Skeleton King is available
– Level 13 cap
– Matchmaking available only with other Starter Edition players
– No Real Money Auction House accessHow do I get the Diablo III Starter Edition?

Log into your Battle.net Account Management page and navigate to the Download Game Clients section. The Diablo III Starter Edition will be available for download there.
Players without a Battle.net account can create one, and then download the Starter Edition.
Note: The Diablo III Starter Edition will not be available without a Guest Pass until 30 days after the game launches.
How do I download the Diablo III Starter Edition?

Navigate to the Download Game Clients section of your Battle.net Account Management page and click the icon for your operating system under the Diablo III heading.
How do I upgrade from Starter Edition to the full version of Diablo III?

Log into Battle.net Account Management and select your copy of Diablo III Starter Edition on the Game Account list. On your Diablo III Starter Edition Game Management page, you will see your upgrade options on the right-hand side.
