As taken from Gameinformer.com...
As 2005 came to a close Nintendo's Satoru Iwata stated that over 5 million Nintendo DS units have been sold in Japan. These sales numbers are attributed to such games as Train Your Brain, Nintendogs, Animal Crossing: Wild World, and Mario Kart DS.
This marks the Nintendo DS as the fastest selling console in Japan, reaching the 5 million mark in under a one year period. Currently the Nintendo DS is outselling Sony's PSP in Japan 2.5 to 1 overall.