PS2 is over 120 million shipped worldwide and that's been out, what, about 7 years? That works out roughly 17.14 million per year if you just divide it up...Gives a good indicator for how the DS is doing, doesn't it?!
DS is coming up to its 3rd birthday (end of the year in Japan). So just for the sake of it, say 3 yrs. That puts the DS at ~15.67million per year using the same simple division...and the DS has increased massively in terms of popularity and is going ridiculously strong in all territories as the PS2 starts to slow down. It's going to be a close fight! First stop is topping the PS2's 20+ million total in Japan (should happen later this year).