A guild on Area-52 is the FIRST Cross-Faction guild in the history of WoW

 Due to some voodoo magic and a buggy glitch, is able to invite both Horde and Alliance members to the guild, they both can chat in guild but you obviously can't understand the other faction!

• An Alliance Guildy
• A Horde Guild Member

If you take the time to look at the guild roster, you'll see a few more (specifically on the second page going by highest to lowest level) Alliance characters.

http://us.battle.net/wow/en/forum/topic/6412211884?page=1