The answer is Yes.
While the answer to this question is a matter of Blizzard policy and can change at any time, Blizzard *is* graciously allowing us to multibox Diablo 3 with basically the same status quo as with World of Warcraft.
That is, they will "tolerate" multiboxing to the extent that you are Good Citizens (see Is ISBoxer allowed? for some reading on that), and even when Warden is enabled, they will not be targeting Inner Space or ISBoxer.
This is an unofficial answer, which is the same as we get for pretty much any game. For legal reasons, game publishers do not come out and officially "approve" of software they do not exercise control over. For example, they cannot legally guarantee that the software does not and will not ever contain functionality that they would disapprove of.
But you said it can change any time
It can. Same with World of Warcraft. Same with pretty much any other game we multibox that has a Terms of Service/Use/Whatever they want to call it at the time.
At this point, Blizzard knows me (and ISBoxer customers as well as other multiboxers) well enough to not just flip the switch, so to speak, and ban us without prior discussion. They COULD if they wanted to, obviously, but somebody would have to seriously provoke that.
How do you know? They didn't give an official word!
I have maintained a positive relationship with Blizzard for several years, and I asked the right people for the unofficial word before investing my time in supporting sandboxing Diablo 3. The team that will be banning people from playing Diablo 3, frankly, has far more important things to worry about than whether or not you are playing more than one account. More important things such as account hacks and fraud.
Some of you may not realize this, but there is a bit of legal history between me and Blizzard. You can find information on the internet if you really want, but the real point is that those days are long gone, and these days are much better for everyone involved. Blizzard wants more customers that they don't need to ban, I want customers that are not at risk of being banned, and, well, none of you want to get banned. That is win-win.
But a Blue said that he didn't know it was possible to do on one computer and that it is certainly allowed on multiple computers
That's cool. So he wasn't aware. Did he know that it's possible to do on one computer using common virtual machine software? What's the difference to this particular Blue whether someone has four virtual machines or four physical machines? (Besides that the guy with four physical machines probably has more money to spend on additional hardware?)
And lastly... I recognize that some people will probably enjoy doing this for the real-money Auction House... It remains to be seen how multiboxers will affect that, but I think everyone is hoping that the Auction House is viable for casual players to sell things too. They didn't build this thing just so a few big names can monopolize, so we'll see how that goes.
That should pretty much cover it. Let's have some fun.