Note that some DNS providers hack around the issue (like CloudFlate by pretending your CNAME was in-fact an A record http://blog.cloudflare.com/introducing-cnam

Note that some DNS providers hack around the issue (like CloudFlate by pretending your CNAME was in-fact an A record http://blog.cloudflare.com/introducing-cname-flattening-rfc-compliant-cnames-at-a-domains-root http://blog.cloudflare.com/introducing-cname-flattening-rfc-...), but if you're self-hosting DNS or your DNS provider doesn't do any special handling, then you can't have a root CNAME