Have you noticed that since the release of patch 4.1 you've seen a lot more players from your realm when queueing via the Dungeon Finder? It's not just a coincidence, according to Zarhym.
Since the release of the Dungeon Finder, Blizzard has received a lot of feedback on it ranging from queue times to who they're queued up with. With the launch of Cataclysm, Blizzard changed the system to be region-wide rather than just on your realm, primarily to help alleviate long queue times.
However, players "miss the feeling of community when they're paired with strangers from other realms with whom they might never speak again," says Zarhym. So Blizzard has updated the Dungeon Finder with patch 4.1 to prioritize matching you with players from your realm, at least initially.
Zarhym says that it was a late addition to the 4.1 patch notes. It was finally added to them last night, stating: "The Dungeon Finder will attempt to place players from the same realm in a party together."