This has been requested multiple times, and been [denied just as many][1] on Meta Stack Overflow. At this point, I doubt it will be implemented with this many times being [meta-tag:status-declined]. You can always create a private chat room, or see [other ways to contact user][2]s.

If you look at that post, it's been requested about 15+ times to no avail, and has been consistently requested since the site started. 


  [1]: http://meta.stackoverflow.com/questions/431/any-way-to-send-a-personal-message-to-another-user
  [2]: http://meta.stackoverflow.com/questions/57537/how-do-i-contact-other-users