A customer just asked about the new conversation view in OS X Lion’s Mail.app.
The messages are all displayed together, and there is a link to display the related content at the bottom of each message.
There is a menu option under the View menu, “Show Related Messages”, which will put your sent items in-line.
By default, the display of related messages is fleeting — as soon as you select a message outside the thread or a different mailbox, the related messages no longer appear, even if you reselect the original message.
If you prefer Mail to display related messages all the time, no matter which message you select, choose Mail -> Preferences, click Viewing, and select Include Related Messages.
Note that this is a global setting; you can’t turn it on for just certain mailboxes or messages.
Some of this post was found on Taming OS X Lion Mail.