Customize Mail Notifications
Happily, this is available for Mountain Lion and Lion users, via Herald, a Mail plugin by Erik Hinterbichler. (What a name.)
It’s a lovely plugin. I’ve been using it for a while now without issue. Some features I love besides enabling notifications for specific mailboxes are customizing the notification window appearance, acting on messages without going to Mail, having messages appear as they arrive or every 5 minutes or in intervals that I choose, opening messages when neeeded, scrolling through the entire message in the notification window, and resizing the notification window.
This is the type of plugin you expect to pay money for and it’s free. Hinterbichler, we love you.