Blizzard has always incentivated "modding", warcraft II map maker, starcraft campaign Editor, Warcraft III WORLD Editor, Diablo 2 set in .txt Tables made to be modded, also Starcraft 2 will include a very powerfull editor itself.
Blizzard also never disaproved Diablo 1 mods...
And they know that editions keep the game running longer ( DOTa for Warcraft 3 ?! )
I dont see Diablo 3 be launched with a kind of editor, but yes to beeing "moddable".