Well, the question "Is Diablo a roguelike" (or was it a roguelike ever) can be answered more easily than with other games.
Of course it once was inspired by roguelikes, but it always was done in a way which you would today call "casual". The Diablo series is, in my opinion, a "casual roguelike-like". It sacrifices thoughtful tactics and the typicals roguelike's many interaction possibilities for two clear concepts: Killing monsters and hunting items in a randomized world. Killing and item-hunting for the sake of its own, without much distraction.
But that's okay, because it IS fun. I didn't play Diablo 3 and probably won't, because you always have to stay online. But Torchlight and Torchlight 2 will surely be on my computers.
In fact, Blizzard games where one of the most important sources of inspiration for LambdaRogue. LambdaRogue is more inspired by Diablo and World of Warcraft than by roguelikes, both in terms of interface, story, and game mechanics (Well, perhaps that's the explanation why LambdaRogue does not appeal to "true" roguelike players, but gets its players from other places
Yesterday I played with a soldier and it was a VERY Diablo'ish feeling *gg* )