Blizzard has announced its intention to activate the Mists of Pandaria pre-launch patch on August 28th, coincidently, the same day that Guild Wars 2 officially launches.
Whether this is actually an attempt to subvert sales of the Guld Wars sequel MMO, or if Blizzard is merely doing it as a bit of friendly competition isn't particularly clear, but the dates are clearly linked. If it is the former, it seems unlikely that it will make a massive difference, since Guild Wars 2 is a brand new, fully formed game and Mists of Pandaria, no matter how big, is just an expansion.
However, the actual expansion isn't released until September 25th, suggesting that most of the pre-launch patch's 6GB of content won't be accessible until the game goes live - but we'll have to wait and see.
World of Warcraft has been bleeding players left and right over the past year, dropping several million subscribers, especially in the past few months, when many are thought to have migrated to Diablo III. Blizzard is obviously worried that Guild Wars 2 will present a similarly distracting gaming experience, enough to perhaps tempt a few long time WoW players away. However everyone is expecting Warcraft's player numbers to get a nice boost as the expansion is released, bringing back old players and perhaps even a few new ones.