It's important to take these early dings with a healthy dose of skepticism, as in the past they've been accompanied by swift action on Blizzard's part when the leveling method has been discovered to be exploitative. Some high-profile people in Cataclysm took park in an exploit, only to find themselves slapped with a level decrease back to 80.
We'll keep you updated as we learn more about this developing story, but if this is a legit power leveling, then it's been done extremely fast. Mists only released in the EU a little over four hours ago.
Updated 1:30 a.m. EDT 9/25/12:
Apparently he's been rolled back to 87, which follows what many in the forums are claiming (although there hasn't been any evidence until now).