Blizzard officially announced they are forming a team to develop a next gen MMO so we know that's in the cards. Based on the evolution of the game elements from Diablo 2 (think talent tree, mana/health, mobs, loot, questing, small group multiplay etc) to Wow (all the same, just bigger, more peeps and 3d), I have to think that they will keep this "game framework" and just continue to push it to the next level.
Put this framework on the darker world of Diablo with plenty of lore, put on a nicely advanced graphics engine, make more involved and interesting quests and there you have it. There will have to be other innovations to draw people away from Wow, but my point is that I don't think we can EVER expect a single player Diablo 3 as the days of yore.
Wow will get stale due to the limitations of the current framework and Blizzard will be ready with another MMO to stay on top. There must be overlap so that someone else doesn't come along and eat their lunch. Even if Wow lost 75% of the current subscriber base that leaves a couple million so I'm sure the servers will be up for many years to come.
Best guess - Order of releases:
Starcraft 2 - Q208
WotLK Wow expansion - Q408
Diablo MMO - Q1 2010
A Starcraft MMO would be interesting, but do they have the lore for it?
Warli,Skywall