Valve hired IceFrog, a developer of Warcraft III mod Defence of the Ancients a while back, and since then there have been persistent rumours that they might be making the mod as a full game. This then seemed far more likely with the surprise announcement of Alien Swarm a couple of weeks back, having hired the mod team who made that too. And now a tweet from voice actor Jon St. John seems to confirm the existence of a DotA project.
Jon St. John is the gravel-voiced man behind those Evil Dead-nicking Duke Nukem remarks. If voice actors are being hired on the DotA project, then you have to wonder at the scale of the project. Perhaps Valve will be moved to reveal some news by the leak... Ah ha ha ha.