This CNAME description is copied in part from the BIND docs [isc.org], hopefully someone with a deeper understanding will straighten us both out! ;)
The Canonical Name resource record, CNAME, specifies an alias or nickname for the official, or canonical, host name. This record must be the only one associated with the alias name. All other resource records must be associated with the canonical name, not with the nickname. Any resource records that include a domain name as their value (e.g., NS or MX) must list the canonical name, not the nickname.
<added>Another definition here [menandmice.com]. Looks to me as if a CNAME can only point at one IP.</added>