The local 2K Games branch has heard back from the mothership about the mysterious disappearance of the publisher's titles from Valve's Steam service.
Apparently the dropped catalogue is a temporary issue, and users can expect the games to return soon. No exact date or precise explanation, but it's good to know we won't be going without. The update was also shared on 2K's official forums (thanks Glen).
It should be noted that the publisher has yet to provide a reason for the Civilization IV price hike. I'm hoping the increase doesn't magically appear on 2K's other offerings when they return.
Update: 2K Games officially has no comment on the price rise... which can be roughly translated as "Suck it, Australia".