The folks from SteamDB
have done some digging inside Steam's files and found indications that Dota 2 may get an update to Source 2 in the near future. Specifically, the info reveals both a 32 and 64 bit version of Dota 2 running on Source 2, along with Mac and Linux support and language packs. A similar update
later added Source 2 Dota 2 beta configurations and support for DirectX 11.
Since Valve's been tight lipped on the Source 2 rollout, we can only speculate when will it arrive to Dota 2, but with The International looming ever close, it seems like a good opportunity to show off the new engine.