I'd like to call your attention to a new feature that we rolled out earlier this month. You can now provide us with a configurable Reverse DNS record for any of your Elastic IP addresses. Once you've supplied us with the record, reverse DNS lookups (from IP address to domain name) will work as expected: the Elastic IP address in question will resolve to the domain that you specified in the record.
If you are using an EC2 instance to send email, you'll appreciate this one. Some other types of applications and protocols (FTP and Secure FTP come to mind), can also benefit from it, but most of our customers have asked for it after they tried to send email from Amazon EC2.
You can provide us with your Reverse DNS records using this form. We'll set up the mappings as quickly as possible and we'll send you an email once everything is all set up.
We count on our customers to provide us with the feedback needed to assign the proper priority to this and to other features. We're always happy to hear from you; send your feature requests to firstname.lastname@example.org and I'll make sure that they are routed directly to the proper team.