Greg "Ghostcrawler" Street has acknowledged on the official forums that there's a bug with the Frost Witch's Regalia Tier 10 set for Elemental shamans. It looks like using the two-piece set bonus in cross-server instances puts the Elemental Mastery Skill on a seven-day cooldown.
"You may not want to use the set while using Dungeon Finder until we know for sure what the problem is. When we have an update, we'll be sure to let you know," Ghostcrawler said. He also noted that there have been a few similar bugs, such as the hunter ability Call Stabled Pet.