Actually, it seems the situation with the mod support is more complicated. From what I gather from interviews, they say that some mod support will be there, but with limitations. It will be possible to mod only certain aspects of the game, but not others, and they will reorganize a little to make more stuff moddable. Also, current D2 mods won't work.
@Rush I wanted to make a comment about this on the D2R FAQ on the frontpage (the FAQ is great, btw!), but somehow the comment box does not show up for me?