While the August NPD numbers show Batman: Arkham Asylum had sold 593,000 copies, those numbers don't include the two weeks of September we've had since then. Factor those in and the game's sold almost two million copies.
The figure was disclosed as part of an LA Times report on some restructuring going on at DC Comics, and we'll presume was provided by Warner Bros, who as a publisher can get hold of timely sales data without too much hassle. We'll also presume that the figures are worldwide sales, and not just for North America.
Nice to see a DC Comics game not only get it right, but be rewarded for doing so.