Will Diablo 3 use Steam?

Diablo 3 will not use Steam or Steamworks: like all Blizzard games, it will use Battle.net. And while Blizzard has stopped short of saying it's a Battle.net digital exclusive sale, the press release for the Diablo III release date states (emphasis mine):

Starting that day, gamers with a thirst for fast-paced action and adventure will be able to purchase Diablo III at retailers throughout the United States, Canada, Europe, South Korea, Southeast Asia, Australia, New Zealand, and the regions of Taiwan, Hong Kong, and Macau. In addition, gamers in the regions above as well as in Mexico, Argentina, Chile, and Brazil will be able to buy Diablo III digitally via Blizzard’s Battle.net® website.

Given that Starcraft 2, Warcraft 3, and World of Warcraft are all Battle.net digital exclusives and are not available for purchase on Steam, it's unlikely Blizzard will offer Diablo 3 there.

I guess not, because neither Starcraft 2 nor any other Blizzard game is on Steam. But you can buy it online from Blizzard's Diablo 3 Battle.net Site, which is probably the reason why it isn't on Steam: why should they pay Steam if they have their own online shop?