mail.domainalias.com

Setup domainalias.com as a domain alias to mydomain.com.  In order to create mail.domainalias.com, do the following:

Within the dns records for domainalias.com, create a subdomain of mail.domainalias.com and point it to http://mail.google.com/a/mydomain.com. This is NOT a CNAME for mail. It is a real subdomain.

Trying subdomain for mail.mydomain.com and see if this gets past the problem of CNAME of mail pointing to ghs.google.com not working if mydomain.com is forwarded.