As posted by MMO-Champion it looks like 5.1 may happen really soon! Several quests talks about how 5.1 should be released 2 months after Mists of Pandaria (September 25) which would mean this week.
But please bear with us for just a while longer and hang on tight to your seat because it will be here before you can say SuperMurlocalifragilisticexpialidocious.
How soon? (fingers crossed) Very, very soon™ ;)