Because EC2 instances are publically accessible on the internet, the postgres (and root) passwords are set randomly on first boot for security reasons. This is also true for mysql where relevant.
You can find the password in the ec2 console output, and in /root/README. This will be changed in the upcoming release to be more user friendly, details to follow...
postgres password set randomly on ec2
You can find the password in the ec2 console output, and in /root/README. This will be changed in the upcoming release to be more user friendly, details to follow...