Update Nov 2012:
DKIM For The Masses
You can set it up for your own Google Apps domain (if you are the domain admin) using these instructions.
It’s a simple process but the trickiest part can be creating the DNS TXT record (which contains your DKIM public key), depending on how you manage your DNS. If you are serving DNS directly via your registrar, Google has some specific instructions for popular domain hosts.
Checking your work
Here’s a quick tip how you can check to make sure you created the record properly and it is being served…