DKII on 22 August 2007
Wii won't be available for as long before the next-gen start. PS2 had 5 years before the 360 and 6 years before PS3/Wii. Wii won't be out that long before the next Xbox if it dominates as completely as the PS2 did (which it would have to in order to get close in sales). MS would launch their next console within 5 years of the 360 launch if the 360 died off as hard.
