It would be simply rude to say 'isn't this the kind of thing that every MMO in its right mind should have at launch?', so instead let's just celebrate that, finally, friends from different World of Warcraft servers can band together to give some sort of huge boss with a repeating attack pattern what's coming to him/her/it/them.
It's been mentioned before, and there's a good chance it's going to carry some sort of additional fee once it's ready for full-on primetime, but let's worry about that later. For now, let's welcome the free beta, which brings cross-server grouping to 5-player regular or Heroic dungeons. The catch is that you and whoever you want to group with need to have RealID-enabled accounts, as well as being RealID chums.
More details on how to make it all happen here. Oh, how I wish this had been around back when I was sacrificing my every minute to the dark god of Warcraftery. Almost, almost tempts me back, knowing I could have funtimes with scattered chums. Only they'd be 5 levels higher than me and with the kind of loot I'd have to kill kittens to get hold of in a hurry.