Recently Google has just added DomainKey Identified Mail(DKIM) feature to Google Apps domain.
You may refer to the step as below to enable the DomainKey.
1. Log into your Google Apps account with http://www.google.com/a/mickgenie.com where mickgenie.com should be your domain name.
2. Click on the Advanced Tools.
3. Click on Set up email authentication (DKIM).
4. Click on Generate new record.
5. Then click on the Generate button.
6. You will get the information as above and may update it accordingly to your DNS editor.
7. Click on the Start Authentication and you are now with the DKIM enabled.