For the first time, World of Warcraft subscriptions have dipped below the ten million mark. According to Blizzard-Activision's latest financial report, the subscriber base currently sits at 9.1M.

As of June 30, 2012, Blizzard Entertainment's World of Warcraft remains the #1 subscription-based MMORPG and had approximately 9.1 million subscribers⁴

Part of the reason for the decline, according to Mike Morhaime, came at the hands of Blizzard's other huge title Diablo III:

"Historically we have seen usage decline towards the end of an expansion cycle. We saw a similar drop in subscribers in the months before Cataclysm, followed by a substantial return of players around the Cataclysm launch. We're also seeing that a number of players took a break from WoW to play Diablo III," Mike Morhaime explained to investors in the most recent financial conference call. Source

Check out Blizzard financial reports.