Even though Blizzard is ending the Diablo 3 beta on May 1st, people continue to play and thanks to that Blizzard may have accidentally revealed a free to play version of he game. Yesterday the beta servers were quickly pulled down after everyone who had a Battle.net account got access to a full featured version of the beta client called a Starter Edition.
Players of Blizzard games would recognize this term as they have released Starter Editions of WoW and StarCraft II, which act as demos for both games. Adding to the theory of a Diablo 3 Starter Edition is that the client people were playing occasionally prompted them to purchase the full game.
So based on the track record with Blizzard’s other two games as well as what people experienced while playing this version of the game, it’s pretty safe to assume that if you want to try out Diablo 3 before buying it; you’ll probably be able to.