Starting this June 24th and lasting for two weeks, Horde players on the servers Firetree, Frostmane and Bloodscale will be eligible to transfer from Horde to Alliance for free. And to prove their seriousness, Blizzard is going beyond the usual here.
Guild leaders who want to faction-change their guild for free will first need to pay for the initial transaction, but will be granted a refund if they change to the eligible faction within the free-faction-change time frame.
What this is aimed at, according to the post itself, is queue times (especially in PvP) and it's a direct attempt to reduce them. So if you're a Horde player on those servers and you're looking to swap sides for shorter queues, here's your chance, and you can even bring your entire guild over for free.