Warcraft IV has been on the tip of most Blizzard fans' tongues for a while, especially after rumours started circulating in 2008 and more recently, in October.
Blizzard just don't have the bandwidth at the moment though, according to Greg Street. Speaking to Strategy Informer, Street said:
"Well, the World of Warcraft team is busy on World of Warcraft, and we have an RTS team but they are busy with Starcraft II, we don't right now have enough bandwidth to tackle something like that."
That doesn't necessarily mean it's not in development though, Starcraft II was in development covertly for quite a few years.
Source: Strategy Informer