Why can't my ECS service register available EC2 instances with my ELB?

I've got an EC2 launch configuration that builds the ECS optimized AMI. I've got an auto scaling group that ensures that I've got at least two available instances at all times. Finally, I've got a load balancer.

I'm trying to create an ECS service that distributes my tasks across the instances in the load balancer.

After reading the documentation for ECS load balancing, it's my understanding that my ASG should not automatically register my EC2 instances with the ELB, because ECS takes care of that. So, my ASG does not specify an ELB. Likewise, my ELB does not have any registered EC2 instances.

When I create my ECS service, I choose the ELB and also select the ecsServiceRole. After creating the service, I never see any instances available in the ECS Instances tab. The service also fails to start any tasks, with a very generic error of ...

service was unable to place a task because the resources could not be found.

I've been at this for about two days now and can't seem to figure out what configuration settings are not properly configured. Does anybody have any ideas as to what might be causing this to not work?