During the setup of Gmail through the panel, you select to move your mail (MX) pointers to googles server. DH does this auto for you.
Then when you log into google apps, you must set a custom web mail address for your domain. By default its a gmail.com/xxxxxx
So log into your google apps account, Service settings --> click Mail
Then the first option is to set a custom web address.
I used mail.mydomain.com
otherwise google has some long winded one that you can use.
If i remember correctly DH already sets up mail.yourdomain.com to point through to google, you just have to set it up in google docks.
Once this has been completed and you have let the DNS replicate, you can now web into mail.yourdomain.com and use gmail for your e-mail needs.
i hope this helps.