Three finger swipe switches message in Mail (Updated)
07. Nov, 2009 
1 Comment
On recent MacBooks that feature multi-touch trackpads and run at least 10.5 Leopard, a swipe with three fingers in Mail will select the previous or next message in the current mailbox. Up or left selects previous, down or right selects next.
Update: Jim Dovey points out in the comments that you can do the same thing with a Magic Mouse and the two-finger swipe.
Post Author
If you're using a Mighty Mouse you can use a two-finger swipe left or right to achieve the same thing, provided the cursor is hovering over the Mail window somewhere (although it doesn't matter where). Not sure if this works in Leopard though.