An update on the Diablo III Website reminds users of Blizzard's stance on cheating, which can result in offenders being banned from playing the action/RPG sequel:

Blizzard Entertainment has always taken cheating in any form in Blizzard games very seriously, and that's no different for Diablo® III. 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.