The top-ranking manga in the latest graphic novel figures from ICv2 (for August) is the 17th volume of Fruits Basket (Tokyopop), which comes in at 3rd place. Impressive as that is, the 15th volume actually ranked higher (at 2nd) in December 2006.
Next to show up are the 14th volume of Fullmetal Alchemist (Viz) in 12th place and the 15th volume of Negima (Del Rey) in 23rd. The 15th volume of Naruto (Viz) doesn’t show up until 26th place, which is explained by its 10th place ranking in July. It’s not the only series to pull off the consecutive-month trick; I’m pretty sure both Fullmetal and Bleach (Viz) have pulled it off as well.