This new feature will be part of a premium package of WoW features requiring an extra fee to use. Blizzard says that only the person doing the invites and creating the cross-server party will need to have access to the premium feature for the system to work. Currently, there is no release date, and the service is being described as a complex one to develop. Blizzard is most likely getting out in front of the news sites and datamining sites now, since pieces of this new system will begin to hit the PTR soon and Blizzard doesn't want too much speculation.
I personally think that this system is really cool for players who have an already-established friends base on other servers but don't want to leave their current home or don't have the money to do so and pay for a premium server transfer. For instance, Sacco and I can finally run some dungeons together. You hear that, Sacco? It's dungeon time.
Premium services always receive some type of backlash in the WoW community, and this one will be no different, so for caution's sake, let's wait until pricing information is officially announced before we explode with indignation. This is, however, a feature that many players have been asking for and will be very popular.