Mine

class redhat inherits linux {

   distro => $operatingsystem ?
   kernel => $kernelrelease ?

   case $distro {
      RedHat:
         case $kernel {
            2.6.9-42.EL: exec{ "echo You're on a RHEL4 u5 box." }
            default: exec{ "echo I don't know what RHEL you're on..." }
         }
      Fedora:
         case $kernel {
            2.6.20-2925.9.fc7xen: exec{ "echo You're on a xen FC7 box." }
            default => exec{ "echo I don't know what FEDORA you're on..." }
         }
   }
}