http://revolution.ign.com/articles/707/707161p1.html

Nintendo president expects new platform to outperform GameCube
by Anoop Gantayat

The GameCube wasn't Nintendo's biggest success, but the company is
looking to change its downward spiral in the console business with the
Wii. Speaking to the Japanese press, Nintendo president Satoru Iwata
said that for the Wii, hardware sales "two to three times that of
GameCube is a possibility."

This would put the Wii at between 40 and 60 million units worldwide.
Not quite PlayStation numbers, but it's possible that Iwata is just
giving a conservative estimate.