After many months of bugging the crap out of Steam via the forums, Mass Effect fans can at last rest easy. The expansion has been officially announced as available via Steam, thats both the regular and deluxe editions, and come with the Inferno armour as a pre-order bonus. Rejoice!
http://store.steampowered.com/app/901242/ - deluxe
http://store.steampowered.com/app/901181/ - regular