According to the latest Nielsen video game ratings, the Xbox 360 is the first current-generation console to top the PlayStation 2 in terms of minutes played per month.
According to the graph provided by the Niesen website, the Xbox 360 actually came out on top of the PS2 in terms of overall percentage of minutes used, the first time a current-generation console has managed to top Sony's last-gen console. It went on to lose the lead in June, perhaps due to one particularly active Nielsen family going on vacation or something. That's the thing with the video game Nielsens...it's relatively easy for one user to make a difference. Then again, it could be indicative of a larger trend, so good for the 360!
Microsoft also points out that the Xbox 360 was also tops in terms of active user percentage, maintaining 11-12% of the active users from January through June of this year, as seen in the graph below. Note that the Nintendo Wii has the lowest percentage of active users during the period, even losing to the Gamecube. That's rather depressing.