A records are name-to-IP, CNAME are name-to-name. When you control everything, you're right, no big deal. But in this case, customers control the DNS and Heroku the servers. Using CNAME to point to an A record at Heroku allows them to decide what IP the traffic goes to. If you use an A record, Heroku is SOL if they need the traffic to go to a new IP.

Posterous has a similar problem last august. They urgently needed to change to a new IP, but all their clients were setup with A-records. Painful.

...Heroku is SOL if they need...

Off topic, what's the meaning of SOL in this context?

"Shit Out of Luck"

In polite circles it can also be "Sorry, Out of Luck"

I'm pretty sure the parent is right that it's "shit out of luck". I knew it as a military thing, and the internet seems to agree[1].

[1] http://www.etymonline.com/index.php?term=S.O.L. (one random link)

Ok. So I'm correct in thinking that for my own domains that I manage it boils down to the same thing, e.g. there is no reason to prefer www CNAMEs over domain A records.

Just wanted to check if I'm missing something.

Good question. Is there an AWS limitation in there somewhere?

No, it is a DNS RFC standard behavior: http://stackoverflow.com/questions/3829966/cname-for-top-lev...