During yesterdays Global Developers Conference, which ended yesterday in Paris, Rob Pardo, Executive Vice President of Game Design at Blizzard Entertainment told everyone that World of Warcraft was intended to be completely free.
Rob Pardo explains that the original idea was to let advertisement campaigns cover the costs and as such be completely free to play. But, Pardo continues, when Blizzard researched the MMO-market, they realised it wouldn't be possible.
Furthermore, he tells how Blizzards decision to enter the MMO-market was "Very naive, otherwise we wouldn't have done it."