CNAME for domain.com?

[Joel Daynes]

All-
    I have several domains that point to my machine at home on my 
Comcast cable connection which has a name pointed at it through 
DynDNS.org. I want to CNAME records in the zone files for the domains 
that point to my machine, which is not a problem for domains where I 
have a hostname (i.e. www.mydomain.com, mail.mydomain.com, etc.), but I 
also want to point mydomain.com at my dyndns.org name rather than making 
it an A record pointing at the IP of my machine. However, I can't use a 
CNAME to do this. Is there an easy way to point mydomain.com (without a 
hostname) at my dyndns.org name as a CNAME, or does it have to be an A 
record pointing at an IP?
Thanks-
Joel