The location has changed since Mountain Lion is out, now the Apple Mail settings reside in:
~/Library/Containers/com.apple.mail/Data/Library/Preferences/com.apple.Mail.plist
The Mail messages, rules and Notes are still in the old location:
~/Library/Mail
Just copy the contents to your new computer, and you are done!
Thanks for the hint with the container directory. That did the trick!
Thanks.
Copy all the contents of ~/Library/Containers/com.apple.mail/ seems to me better to have attachments and more.