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...

Unless you use SNI (which is only supported in some browsers) you have to have one IP per SSL host as the certificate is sent before decoding the request. ELB doesn't use that and must have 1 IP per hostname.