Given Blizzard's previous stance on cross-faction communication ("Nope, not happening - use emotes, dude"), this seems kind of odd. While the idea of making friend and chatting with friendly Horde is interesting, I can't help but think that this ends up opening a door to a lot of potential abuses. Battleground honor-farming collusion are and spying on enemy forces are two obvious ones. "Hey, Wintergrasp time. Jump on your Horde toon and let us know what they're planning, OK?" It just seems... wrong to allow players in a PvP environment to potentially communicate with one another.
Hopefully, Blizzard will impose some restrictions on this ability - not allowing cross-faction chat when you're flagged for PvP would be a reasonable one, I think. Or maybe only allowing cross-faction chat if it's also cross-server chat (though that still leads to potential problems in battlegrounds and other cross-server instances).
Personally, I think that implementing some sort of language skill to allow cross-faction communication would be a more interesting proposition. Make it something hard - requiring an achievement like Diplomat - before you can even start to learn the opposing faction's language. Ultimately, it would be more immersive, and allow for more nuances in game play - yeah, that Rogue? He's stealthed and crept into your base, and he understands what you're saying. Mostly. So long as... oh, crud, they switched to Dwarvish!
I mean, if you're going to poke holes like this in the communication barrier, why not go all the way? Cataclysm is going to shake everything up, after all. Why not allow Horde and Alliance to start to learn each other's languages? Or if that seems like too much, then introduce a common third language - Titanic - that can be learned by anyone using the new Archaeology skill.
Interesting stuff. One day, maybe, we'll even be able to group together. Imagine a mixed PvE/PvP dungeon... someplace like Uldum, an ancient Titan city. Powerful devices of the Titans are stockpiled there, magic and technology essential to the continued survival of Azeroth. It will require the combined power of the Horde and the Alliance, working together, to defeat the guardians the Titans have left behind. A situation where cooperation is borne of necessity... at least until the last guardian is defeated, and the only thing standing in the way of claiming the relics of the Titans are your one-time companions in adversity.
Now, that would be a whole lot cooler than being able to say, "hey bro, r u their?" to your Horde buddies. IMHO.