I was bothered by the fact that I could not easily change the state of a message within a conversation from 'unread' to 'read.' That is when one clicks on a thread in the middle pane of mail, one can read all incoming messages in a conversation, but this does not change the state of all the messages. You actually have to click on each individual message.
So playing around, I discovered that hitting the Right-arrow key unrolls the entire list of incoming messages in a conversation right in the middle column immediately under the cluster list item. It is now only a matter of navigation with the arrow key to read, and hence, change the state of message, and then come back to the original conversation cluster item. You can then close the list by hitting the Left-arrow key.
[crarko adds: This seems like a pretty handy shortcut.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110818073932356