Note that if you're cnaming to some thing that is doing dns based load balancing or Geo targeting, introducing flattening in the middle may reduce the ability of your provider to do those things.
If you want dns based load balancing on the root domain, you really have to delegate that domain to your DNS based load balancing service.
Otherwise, set up a couple stable IPs to redirect to a subdomain and nothing else. (I'm comfortable putting two quality machines in different data centers for this, but you can use a load balancer it you have access to quality load balancer). If all of your published urls have www (or m) and all of your inbound links have it too, it's not really a big deal if the root domain is unreachable for some time in the event of a server/load balancer/datacenter failure.
