Blizzard has said they'll ban Diablo 3 players permanently if they're caught cheating.
Although we're not sure what sparked the sudden message regarding cheaters, Blizzard is taking cheaters extremely seriously, which is a great concern for those who do cheat due to the game's always-on DRM.
If a Diablo III player is found to be cheating or using hacks, bots, or modifications in any form, then as outlined in the Diablo III end user license agreement, that player can be permanently banned from the game. This means that the player will be permanently unable to log in to Battle.net to play Diablo III with his or her account.
Playing Diablo III legitimately means playing with an unaltered game client. Doing otherwise violates our policies for Battle.net and Diablo III, and it goes against the spirit of fair play that all of our games are based on. We strongly recommend that you avoid using any hacks, cheats, bots, or exploits. Suspensions and bans of players that have used or start using cheats and hacks will begin in the near future.
Diablo 3 is currently the fastest selling PC game of all time with the action role-playing game selling 3.5 million units during its 24 hours of launch, while it went on to sell 6.5 million units