Bruce Phillips and Qwerty Denzel have written an AppleScript that solves one of the most frequently emailed complaints about Mail.app that I receive—how to move by keyboard shortcut to the next unread message.
I usually tell them that the Spacebar is as good as it gets.
However, using this script 
combined with an app like FastScripts , you can cycle through your unread messages in the Preview pane more efficiently.
It won’t satisfy those who want to do this in the Message window like they once did in Eudora, but you can’t have everything.
[Via Daring Fireball ]
Thanks for the tip. For those who tag and act-on mail, though, it doesn’t solve the major problem of moving to the next mailbox that has unread mail. It only does the unread mail in a single mailbox. Sigh…
As a workaround couldn’t you make a smart mailbox with all your unread mail and use this script in that?
[...] Next Unread Message (Applescript to move to next unread message) was added to the Added Functionality section. [...]
Lavar: you are a genius. Of course, with an “unread mail” smart mailbox you don’t need this script at all.