Personally I think it would be great because a) not everybody remembers to leave contact information and b) you get the instant context and can forgo the "hey, I'm not a stalker but I thought you wrote something interesting on HN"
This is really a usability issue. It might be more effective to make it clear what is and isn't private than to build an internal messaging system.
Just add an email address to your profile, like everyone else who is interested in meeting new people.
It sends an email on behalf of the user, and the recipient can reply back to the user via email (the sender's email address appears in the From: field.)
It works pretty well there, and there could be a karma threshold and a setting to disable it here on HN if it were to be implemented.
But it would be nice if yc would email people on your behalf. Most people do not think of adding their email address to the profile area.