A Dervish was originally one idea behind a possible class in Diablo III.
One of the proposed character concepts for Diablo III was the "Dervish" class. This was rejected, for reasons
Diablo 3 Lead Designer Jay Wilson said:  explained:
- One of the names that people on the team liked was “dervish.” They thought, “Aw, there’s never been a dervish in a game,” and I said, “What the hell is a dervish?!” I mean, I know what it is, but it’s a little obscure. So we said it’s fine to take a class and twist the visual expectation or the functionality expectation, but naming wise you should probably give people something that they can really chew on, because that might be the first thing they ever see. And if they’re going to be interested in the class, it’s better not be something that they’ve never heard of.
It's possible that we might see this class in the game, but it wouldn't have "dervish" for a name, since as Jay Wilson pointed, out that's too obscure a class name. They want short, clear names with a connotation players are familiar with. The skills and back story can be very creative and interesting, but the name should not be so odd as to confuse players.
What's a Dervish?
It's not clear from the name (which was Jay Wilson's whole point) what a Dervish character would have been like, since the actual definition of the term is a sort of ascetic holy man of the Muslim faith, who lives under extreme vows of poverty. The term "whirling dervish" has some familiarity in common conversation, and could perhaps be connected in the game to a Whirlwind-like Barbarian attack. However the literal meaning has nothing to do with violence of combat, but to a sort of spinning body movement to induce a dazed, trance like state.
Obviously, Blizzard's (considered and rejected) use of the term for a character class wouldn't have had much connection to the dictionary definition of the term.
You can read more about this, and confirmed classes among the other classes of Diablo III. You can also find more media coverage in the wiki as well as a Diablo III Basics page, with a fact sheet of all known info about Diablo III.
Dervishes in Diablo III
In the end, Blizzard did add Dervishes to the game, in the form of the vicious Desert Dervish.
The D3 Team is obviously less worried about confusing players with a monster name, since they are generally killed pretty quick anyway.