There’s no direct intent to say that we don’t want D3 to be moddable. And, to be honest with you, the technology itself, on just a fundamental level, makes this the most moddable version of Diablo there’s ever been.
However, we have these other goals that supersede modding; we want to provide a safe and secure experience for players to play in and trade items in, and in order to do that, we had to make the game online play only. Once we made that decision, that effectively eliminated the possibility of having moddable games, since you’re going to have to connect to our service in order to play. So that’s a slight consequence of our online-only decision.
I’ll say that there’s never really been an intention with past Diablo games to make them moddable, either; it’s just that people found a way to make it happen. It’s not necessarily something we went out of our way to support.
While this seems to be the same case we saw with modding in Diablo II, there is one catch: the lack of any LAN support, and the requirement of a server to handle many basic game mechanics, means that anyone who actually manages to create a mod will have to violate Diablo III's end user license agreement. Perhaps someone will find a way around this, but until then, it's all speculation.
If you want to learn more about how modding has affected the series, we encourage you to read one of our March articles, Blizzard's "Anti-Modding" Stance: Another Look, which features insights from renowned Diablo I and II modders, some lesser-known details about the Diablo team, and a great deal of history and speculation.
For discussion on this topic, please see Rhykker's thread.