According to Blizzard, faction interest in queuing for player-versus-player servers is the most significant contributor to long queue times. So the studio is trying to alleviate those delays with a limited test of a free faction change. Blizzard's first experiment will run on one realm, Barthilas, where eligible characters will be able to switch from Horde to Alliance for free starting on May 27. The developers want to keep the test going for about a week, but will extend or shorten that period depending on how things go.
A faction change usually costs $30 in the Battle.net Shop. Guild leaders can also take advantage of this offer to switch over their entire guild; they'll have to pay the regular fee at first, but as long as everyone in the guild completes the change within the free period, Blizzard will refund the fee.