We are pleased to announce that Elastic Load Balancing now supports automatic re-registration of back-end instances when they are stopped and restarted. You can reduce costs by using the ability in Amazon EC2 to pause your EBS-backed instances when they are not needed. With the automatic re-registration functionality,when you restart a stopped instance that is registered with a load balancer, it is automatically brought back into service with the load balancer.
Source: amazon.com