We are investigating increased API error rates for EC2 Spot-related APIs in the US-EAST-1 Region. Affected customers will not be able to launch new EC2 Spot instances, but existing instances are unaffected. Launching new EC2 On-Demand Instances is unaffected. We have identified the root cause and are working towards resolution.
from Amazon Web Services Service Status: http://status.aws.amazon.com/