Last I heard, they are working on letting the regions mix at some point, but because the decision was made late in the development cycle, they didn't have time to implement it at ship. The first priority is allowing the Australian players access to the US Battle.net servers, as initially they are region locked to the Asian servers.

Part of it may have to do with the work involved on the Battle.net side. They need to find some way to allow cross region friends, and that involves allowing cross game communication into WoW. With the crunch to finish the next WoW expansion, they may be holding off till later to complete that piece.