In a rather unexpected move blizard has now allowed players to group up for Siege of Orgrimmar Normal and Heroic crossrealm! This change is already live in the US and should go live in the other regions in the next few days.
Quote from: Blizzard
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.
Ion Hazzikostas (Watcher) clarified a few things:
Quote from: Blizzard
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.