I believe the Suez- and Panama Canals both qualify as terraforming in a Civ3 sense. Although they have not shifted huge amounts of earth, they have a very significant impact on the movement of units through continents instead of around them.
I could add the Aral Sea in central Asia as another significant act of real-life terraforming (or in that case, more like terror-forming). What was once a stable inland sea with verdant pasture surrounding it, is fast becoming a dust-blown desert and salt pan because intensive upstream cotton farming diverts most of the water flowing into it.
There are now several huge open-cut mines in various parts of the world, the volumes of which compare to mountains.
Doubling the CO2 content of the atmosphere over the space of just 100 years is also terraforming. The effect on global average temperature may take a while to become fully obvious, but if the predicted outcomes are realized, we can expect to see great changes in coastlines, deserts and polar ice coverage.
Let's not also forget that it wasn't too long ago that some people were advocating the "peaceful" use of nuclear explosives to create harbours, level mountains etc. So, it's not that mankind lacks the capability to do this stuff, they just decided not to (for now).
My 2 cents..