Looks like Valve Software is indeed working on a standalone version of Defense of the Ancients.
Over the weekend, Duke Nukem voice actor John St. John posted a message on his Twitter page revealing that Valve Software is working on a project related to the popular Warcraft III: Reign of Chaos mod, Defense of the Ancients.
"Had a great time in Seattle last week recording for DotA," he said. "The guys at ValveSoftware are awesome!" Sometime later the Twitter entry was deleted.
The "confirmation" arrives after Valve recruited Defense of the Ancients designer Icefrog last year, hired on to tackle an unnamed project. Although nothing official has been released by Valve regarding the supposed game's development, Superannuation recently uncovered a trademark filed by the company for "DOTA," giving the rumor more substance.
Defense of the Ancients is a custom scenario for Blizzard's fantasy-themed RTS. The object is to destroy the enemy team's Ancients--heavily guarded structures at opposing corners of the maps. Players use heroes and AI-controlled fighters called "creeps" to complete the mission, leveling up and purchasing new equipment in the process.
Recently Valve released a standalone, Source Engine-powered version of the Unreal Tournament 2004 mod, Alien Swarm. The game is free to download and play on Steam, offering a 4 player co-op "epic bug hunt" viewed in a top-down perspective. It's speculated that Defense of the Ancients may also follow in the footsteps of Alien Swarm, offering a free-to-play experience for Steam gamers as well.