Starting with Fedora 17, SELinux prevents sshd to initiate connections to remote HTTP ports (80 and 443). In SELinux terms: sshd_t is not allowed to name_connect to http_port_t. This broke Yubikey authentication on a system with SELinux in enforcing mode, unless a custom SELinux policy was written and enabled.
Based on a bugreport in Red Hat Bugzilla, a boolean was added to the SELinux policy for Fedora 18 and up, that can be toggled to allow sshd (and some other SELinux types) to connect to remote HTTP ports.
To make a long story short, if you want to use a Yubikey on a system running Fedora 18 or higher (and probably RHEL7, eventually), you’ll need to toggle the authlogin_yubikey SELinux boolean, like so:
setsebool -P authlogin_yubikey 1