When the four folks (whose names I don't recall of the top of my head) left Blizzard, some of their comments hint that a Diablo III is being worked on presently. That doesn't really give a timetable, and Blizzard is pretty lax about deadlines (they can afford to be) so I would say we might hear an official announcement sometime next year. I would guess around April, plenty of time for hype to build up.
World of Warcraft supposedly comes out "sometime in 2004". I would guess they're going to work hard on getting that out for the Christmas season. However WoW and a possible D3 would be worked on by different teams altogether, so the release dates may not be dependant too much on each other.
If I had to estimate, I would say not until 2005 at the earliest. Blizzard tends to announce their games pretty early, so even if they announced it next month, it'd be fairly safe to say we won't see if until then.