Blizzard does not want it to be moddable
has apparently decided that the upcoming release of Diablo III will not be supporting any kind of mods or modding. This is simply because Blizzard does not want modding and they have other goals in mind for the title.
The most important goal for the title is that they want the title to provide a safe and secure experience for players. With this goal in mind, modding had to go because the game will require a connection to the Internet to play.
According to Blizzard, they never really intended for previous Diablo titles to be modded or to support modding, but people just figured out a way to do it and then they made it happen. It was not something that the company endorsed.
Diablo III is on track for a release in early 2012, and it is assumed with the comments from Blizcon that a console version is coming; but it will likely arrive after the PC and Mac releases of the game.
You can read more Blizzard’s Julian Love here