Cross-realm raiding now enabled on all servers
This change is currently live.
If your cross-realm Flex group has been doing well and defeating Garrosh, now might just be the time to step into Normal mode, test the waters against the forces of Hellscream on a higher difficulty, and reap all the rewards for doing so. Good luck!
Edit: Ion "Watcher" Hazzikostas has stepped into the thread in question to clarify a few points. Read on after the break.
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.
