You're absolutely right that it's unclear on the pricing page - we still have some polishing to do. By "piggyback" I mean SSL on your application's default URL

You're absolutely right that it's unclear on the pricing page - we still have some polishing to do. By "piggyback" I mean SSL on your application's default URLs which are of the form FOOBAR.dotcloud.com. Unlimited means that, no matter how many domains you host on dotCloud, the price remains the same. This is especially generous since, behind the scenes, each new SSL-enabled domain requires a dedicated IP, which costs us an extra $20-$30/month on EC2.