Within the freshest Starcraft 2 panel from BlizzCon 2008 Blizzard proclaims, that Starcraft 2 will be expanded into a trilogy. Furthermore Blizzard verifies, that Warcraft 4 will be released with the same principle.
I saw this on one of blizzards things, and this is on most fansites I know of..
Its confirmed but not announced... Guess what? All those people from those wc4 debates that thought wc4 would never happen? Guess who was proven wrong?
Not only this there have been many hints in reviews that its up next. And why not? Just because WoW is around doesn't mean they can't make a RTS, theres still a lot to explore in warcraft, sure illidan and LK are dead or will be, but theres still a lot on the table to use.
Plus on one of the reviews they said if they did a wc4 they would ignore WoW.