Blizzard has announced that World of Warcraft players can now invite their Real ID friends to join a party and play through a 5 player normal or Heroic dungeon together, regardless of which servers the friends' characters on. This is a big step in improving the social aspects of the MMO giant, as players who have previously been unable to play together will be able to join forces for the first time. The service is just undergoing public testing right now, though, so there may be a few issues to iron out.
There's a couple of restrictions on the service: players who want to play together across servers must have Real ID enabled, and both must be Real ID friends. Players can invite a mix of Real ID friends, character-level friends, guildmates and random players to join the party, though -- with the Dungeon Finder filling in the gaps when necessary.
Public testing has begun, but Blizzard hasn't yet determined how long the testing process will be. The company also hasn't yet decided which aspects of the Real ID Party system will be premium-based, and promises an announcement on this towards the end of the testing period, whenever that will be.