I tried this out with my preferred everything-launcher, Quicksilver, and it worked like a charm. I saved the script in Script Editor, then simply added it as a Trigger in Quicksilver with a keyboard shortcut. This has the fortunate side effect of being accessible from anywhere in the OS, so when I press my hotkey, Mail comes to the front and moves to the next unread message in the selected mailbox. Here's hoping Apple's Mail engineering team is listening and has something a little more integrated cooked up for Leopard's Mail.
[via Hawk Wings]