People has different budget for handhelds and consoles. I am willing to pay $200 for a console does not mean I will pay $200 for a handheld. So if DS win PS2 it may still be on fair ground.
WW Market share prediction on 31st March 08 (I make this prediction on 13 Dec 07):
Wii - 50%
X360 - 30%
PS3 - 20%