How do I setup Postfix (Ubuntu) and Google Apps to DKIM sign outgoing emails?

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http://serverfault.com – I am using Linode and have built a LAMP stack. I setup Google Apps (free edition) to handle all incoming mail. I've also setup Google Apps to use DKIM authentication. The problem is when my application sends mail (sign up verification, lost password, etc.) the email is not signed. I do know that the DKIM authentication was setup correctly because when I send mail from Google Apps it is signed as expected. How do I go about signing application generated emails via Google Apps DKIM authentication? I am in no way a server admin, so detailed instructions would be very much appreciated. (HowTos)