We’ve recently implemented an often requested feature to enable cross-realm raiding for Siege of Orgrimmar. Cross-realm BattleTag™ and Real ID friends can now raid together and take the fight to Garrosh on Normal or Heroic difficulty. Players will be able to earn greater rewards such as Reins of the Kor'kron War Wolf or the title of Conqueror/Liberator of Orgrimmar; both of which aren’t available to be earned within Flexible or Raid Finder difficulty.

This change is currently live.

This is a change very late into a tier, which leads me to believe that when WoD comes out this change won't be live and will still require guilds on the same server to tackle Mythic raiding.
Correct -- current-tier Mythic raiding in Warlords will still be limited to players from a single server. In the meantime, though, this change may help guilds that are trying to recruit and grow in preparation for Mythic raiding.

As for realm firsts, realm firsts are guild achievements; you need to be in a guild group, which is by definition 8/10 or 20/25 members coming from a single guild on a single server. You could theoretically have a couple of guests from a different server and still claim the realm first, but ultimately 80% of the group still has to be from the same guild.

With the Cross Realm Normal and Heroic raiding, is there any plans on allowing people to trade XRealm as well?!
Nope. Apologies if that torpedoes any hopes of selling Kor'kron Juggernauts to the entire continent.

As for loot and lockout questions above, nothing about this affects either of those. Loot is not personal in Normal/Heroic; the exact loot setting depends on the raid leader, as always. As for lockouts, Normal lockouts will continue to prevent you from entering a raid in which a boss is alive that you've already killed during a given week. Heroic still uses fixed lockouts, and once you're saved to one Heroic ID, you cannot zone into any different Heroic ID that week.

Are the Cutting Edge achievements for Heroic Garrosh still obtainable with this change?
The "Ahead of the Curve" achievements are still obtainable with this change, which is what awards the Reins of the Kor'kron War Wolf.