I had an error (Permission denied (publickey)) while trying to launch an EC2 Spark cluster with spark-ec2. Turns out; you need to have a key pair for each zone (in my case I was using the keypair from a different zone).
Warning: SSH connection error. (This could be temporary.) Host: ec2-xxxx.ap-northeast-1.compute.amazonaws.com SSH return code: 255 SSH output: Warning: Permanently added 'ec2-xxxx.ap-northeast-1.compute.amazonaws.com,xxxx' (ECDSA) to the list of known hosts. Permission denied (publickey).