Blizzard will be continuing its "Starter Edition" trend with Diablo III, beginning with the game's launch tomorrow. To try it out the first 30 days after launch, though, you'll need an invite/Guest Pass from a friend who has bought the game -- time to start sucking up.
If you've played the beta, this demo won't offer you much you haven't already seen: you play the first act up to the Skeleton King, and your level cap is 13. Also, there's no Real Money Auction House, and matchmaking is limited to other demo players.
Back in April, Battle.net users found the Starter Edition had mysteriously unlocked for everyone. Turned out the unlock was a bug, however, and anyone who attempted to access it were sent straight back to closed beta.