They are using Diablo III to fully set up the story for the MMO, and they are making sure to flesh out Sanctuary as much as they can. I mean, why draw a picture of Skovos when it cannot be accessed in Diablo III? Sounds fishy...
And by the way, why would they risk putting all this time into a new MMO with a brand new franchise? People might not like it, so it would be stupid to assume a new franchise would be popular enough to have it's own MMO.
Diablo (or StarCraft) is much more likely to get the next-gen MMO title. Both games are proven to be favorites by fans, and they have almost guarenteed success as MMO's. But Diablo seems the easiest to bend into being MMO.
However, they did take Warcraft III, an RTS, and transformed it into an MMO just a few years after it's release, so it's very likely they could do the same after StarCraft II - which I'm sure would be sweet as hell.