If you roll the pointer over the message dividing line four instant selections popup for Trash, Reply, Reply All and Forward.
This can be somewhat useful and avoids a trip to the tool bar or menus, especially if you are in a threaded message with numerous responses.
I never noticed this behavior before for the Mail client, and thought it was worth a mention.
[crarko adds: I looked in Snow Leopard and couldn't reproduce, so I assume this is indeed new behavior in Mail 5.]

