"Had a great time in Seattle last week recording for DotA. The guys at ValveSoftware are awesome!"
IceFrog, one of the main developers of the mega-popular WarCraft III modification 'Defense of the Ancients' joined Valve Software last year and was put in charge of a project. Back in May, IceFrog wrote that Valve would be "making an announcement soon" with regards to his project, but nothing has been revealed.
Based on John St. John's tweet, it appears likely that Valve is cooking up its own version of Defense of the Ancients, which would presumably be a standalone edition running on Valve's Source engine. Adapting new games from DotA isn't a new practice, with S2 Games releasing Heroes of Newerth and Riot Games releasing League of Legends. Even Chris Taylor got in on the action with Demigod.
Shacknews has reached out to Valve for comment and will update you with any new information.