Close Mail.app on both machines, and then find the file ~/Library/Mail/V2/MailData/LSMMap2 on the old machine and copy it into the same location on the new machine.
This is useful on those occasions when you need to copy or move OS X’s Mail’s rules for filtering spam from one account to another, or from an old machine to a new one when you don’t want to use Migration Assistant.
The same can be done for any Rules (copy MessageRules.plist) or Smart Mailboxes (SmartMailboxes.plist) you’ve set up.
Accurate as of OS X 10.7 (Lion)