Today Amazon Web Services launched two new features in Amazon EC2 that are essential tools in building highly resilient applications: Elastic IP addresses and Availability Zones.

In summary:

• Elastic IP addresses are associated with a customer account and allow the customer to do its own dynamic mapping of IP address to instance. Using this dynamic mapping applications can remain reachable even in the presence of failures. This is an area where for example DNS reconfiguration is too slow a technique.
• Availability Zones allow the customer to specify in which location to launch a new EC2 instance. The world is divided up into Regions and a Region can hold multiple Availability Zones. These Zones are distinct locations within a region that are engineered to be insulated from failures in other Availability Zones and provide inexpensive, low latency network connectivity to other Availability Zones in the same region. By launching instances in separate Availability Zones, applications can be protected from failure of a single location.

With these two features EC2 customers now have tools to build applications that can tolerate a wide range of failure scenarios.

For more details visit the EC2 detail page and the Forum announcement

Update: excellent articles by the guys at RightScale: Using Elastic IP in switch-over scenarios, using Availability Zones to set up a fault-tolerant site and combining Elastic IP and Availability Zones.