I've got a prototype site built in D6, pertinent modules:
The site is built around a list of open requests; to solicit a private response. These nodes are of content type Request.
All open requests are public in that all users (logged in or not) can view these nodes.
When a response (comment) is received to an open request it should only be viewed by the author of the original request, or the author of the response.
These responses should be shown to these users on the original node as comments and on a seperate list.
How do I do this?
A quick search on D.o. came up with this forum post asking to do the exact same thing: http://drupal.org/node/919914
They suggest using Node Comment and Content Access together to achieve it, but I'm not certain Content Access will give you the specific option you're looking for.
It would be a start though, and you might be able to hook into Content Access and create a new "Parent Node author" or similar option if you are up for a coding session.