From Amazon:

To enable your Amazon EC2 instances to fully utilize the IOPS provisioned on an EBS volume, you can launch selected Amazon EC2 instance types as “EBS-Optimized” instances. EBS-optimized instances deliver dedicated throughput between Amazon EC2 and Amazon EBS, with options between 500 Mbps and 1000 Mbps depending on the instance type used. When attached to EBS-Optimized instances, Provisioned IOPS volumes are designed to deliver within 10% of the provisioned IOPS performance 99.9% of the time.

Mousing-over the Instance Type on the Image Configurations page in admin will say whether the EBS-optimised is available for that instance type, but you can't actually use opt to use that anywhere.