1UP: Speaking of the WOW experience, other action-RPG games have attempted a subscription model of sorts, can you talk at all about that for Diablo III?
RP: Well, ultimately, we’ll decide on the final business model of the game in the future, but I don’t think you’re going to see a subscription model in the realm of WOW. It’s a boxed product game, it will be more similar to say, the Diablo II or the StarCraft II model.
A 1up interview. I believe there was another that basically says no, but I'm still trying to find it. Since battle.net isn't charging for Starcraft II monthly, it's not going to charge for Diablo III monthly, or they wouldn't have bothered saying:
It seems to me that they are tying everything to bnet now. Even WoW accounts. I would not be surprised to see them mimic Xbox Live with their Bnet service. Pay us $15/month for access to all of our products, including any MMOs.
I mean, they already have achievements in WoW, and was also stated in an interview that they plan to bring achievements to all of their future titles. They've also been spending the last while "upgrading battle.net"... it gets you wondering.
Of course Xbox Live is what... like $60 per year. Which is roughly $5 per month.