There are 5 C2 games that I know of currently released on Steam - read here
None of them have achievements, so probably don't have the Steamworks API implemented but as has been said, that is just optional - the only real requirements are that it runs from an EXE and doesn't break Steam. You still need to get through Greenlight though.
I believe another HTML5 game that used node-webkit did have Steamworks implemented and there was some discussion about the possibility of making an unofficial plug-in for C2 but I don't think anything came of it.
Steam probably won't be added as a specific export option for C2 until some time after submitting a game to Steam becomes as easy as submitting an app to the Google Play store.
EDIT: found the discussion about Steamworks plug-in -