A few days ago Blizzards chinese manager have released Blizzard's game release schedule calendar.
In this calendar there are some interesting thins about upcoming Blizzards games.
1st Starcraft 2 expansion and Diablo 3 will be released in 4Q 2011.
Also there will be more WoW expansions :) and WoW movie.
But something that don't fit here is there is no Warcraft 4 :(
I don't know why have Blizzard abanded Warcraft 3, they don't even release new maps on hollydays.
But this may not be official Blizzards game release schedule calendar because Blizzards have not officially confirmed this, this may mean that there still is chance for Warcraft 4 :)