With the patch 6.0 pre-expansion period almost upon us (certainly within the month) we know that we'll get to see the debut of Mythic difficulty Siege of Orgrimmar. However, that difficulty will be cross-realm, just as heroic SoO has been up to now. Watcher answered a recent question on twitter about how Warlords of Draenor mythic raids will work in this regard.
@goldman1337 New Mythic tiers will not be cross-realm; that restriction will be removed after some time, likely when the next tier arrives.- Watcher (@WatcherDev) September 30, 2014
What this means is that the newest Mythic raid will always be realm only, keeping the prestige of realm first chases alive, but as another tier is launched, that previous Mythic tier will hopefully become cross-realm, allowing you and your friends (or even just willing strangers, ala today's SoO pugs) to run the older content. Seems fairly reasonable to me.