By rough consensus on the Fedora list, the default user should be "fedora". https://lists.fedoraproject.org/pipermail/cloud/2013-June/002498.html The strong minority view was "no default user at all"; there is also a case to be made for using ec2-user for Amazon EC2. I think we should address the latter two with clear documentation, as is relatively easy to use cloud service metadata to get something other than the default.
This was fixed in cloud-init-0.7.2-3.fc19.