I just launched one of the Ubuntu Server AMI's on Amazon EC2.
I connect just fine, but I don't see anything to tell me the default login credentials.
login as: ?
On EC2 all the Ubuntu AMI's should be connected to like so:
So you connect as the 'ubuntu' user using the ssh key you injected into the instance at launch (not a password). Once authenticated, you can issue password-less root commands with sudo.
(note: your ssh key might be .priv, or .pem, or no extension, this is just an example)
Here are also some handy resources about connecting to EC2 instances:
Launching an instance and choosing or creating a new SSH keypair: http://docs.amazonwebservices.com/AWSEC2/latest/GettingStartedGuide/index.html?LaunchInstance.html
Connecting with your ssh keypair: http://docs.amazonwebservices.com/AWSEC2/latest/GettingStartedGuide/index.html?ConnectToInstanceLinux.html
What they don't mention there is that you connect to Ubuntu instances with the 'ubuntu' account and not root.