Something like this would be really nice, even if it's just something like how Wikipedia does it: http://en.wikipedia.org/wiki/Special:EmailUser/Jimbo_Wales (You'll need an account to visit that page.)

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.