The new Amazon EC2 Elastic IP feature, which provides for a static IP for instances in the EC2 cloud, should resolve a lot of problems which I won’t go into here.
Note, however, that when you associate one of these static IP addresses with a running instance, the old IP goes away. So if you have DNS entries pointing to your servers, you will need to change them at the same time. And I hope that your TTL is low.
It is too bad that Amazon can’t have both IPs associated at least for a little while — like 48 hours so that users with evil caching DNS servers won’t be hosed in the interim.
