Domains that resolve to an IP... I'm not 100% sure how they are configured behind the scenes, but the ELBs have a bunch of physical IPs wired into them, so they can SSL terminate for many domains, something that EC2 instances cannot (currently) do.

So Heroku leverages the ELBs to circumvent the 1 IP address per EC2 instance issue.

EDIT: more info here http://serverfault.com/questions/285680/what-is-the-technolo...