It was Forsaken way back in the first WoW beta.
However, I think the issue with that was that the factions could only communicate, they couldn't actually work together in any meaningful way. If the factions can actually start working together, there might be a few immature brats who insult other people's choice of faction, but they won't be the majority and things will settle down after a few weeks once the novelty has worn off.
Although at first it might be best to limit communication to inside of cross-faction groups just to be on the safe side, and only later open it up everywhere.