I'm glad you brought this up, because just this weekend I've been trying to help my mother-in-law, who's in another state, with a few problems on her system. We're both running ML, and I realized with a shock that there was no option to screen share for me to take control of her computer. In one menu in Messages the words "screen share with XXX" appear, but they're grayed out and not a clickable option.
Google searches only bring up "screen sharing" between two computers on the same network (business, home wi-fi, etc.), but what's missing is sharing between two computers that aren't on the same network.
There are third-party options like Logmein, but surely there is still a native OSX solution for this, right? Let's see who can get us the answer...