In Japan, or worldwide?
Well, anyway.
Dragon Quest: Monsters: Joker sold 1.5M in Japan.
That is a spin-off title.
As the DS seems like the perfect RPG console, I can see it selling more than the Ps2 installment. I'm calling 4M in Japan (about equal to DQ7)
DQ is also picking up out of Japan. DQ:M:J sold 500K in Americas (highest the series have seen). We lack Others data on it, but we know DQ is about as big in Others as Americas.
So I say it will sell 1.5-2M out of Japan, and 4-4.5M in Japan. 6M WW, being S-E's best selling single-plat title this gen (I doubt it will beat FFXIII Ps3 + X360 combined though, but maybe).
http://www.vgchartz.com/games/userreviewdisp.php?id=261
That is VGChartz LONGEST review. And it's NOT Cute Kitten DS