Blizzard has just announced in a post on the Battle.Net website, that Diablo 3 will come with a starter edition. Now in order to get this edition you will need to have a friend or know someone who purchased the game. This is because each game will come with a limited number of guest passes, which will let you in to play this new edition.
Since this is basically a trial, you will not get access to most of the good parts, that Diablo 3 has to offer. You will be limited to Act 1, you will not be able to level up past 13, you will be limited to only playing with other starter edition players, and you will be restricted from using the real money auction house. Players who have no friends will have to wait 30 days until the service becomes available for everyone.