Not really such an outlandish prediction like some have reacted.
Wii will be at least 43m by this years end. Combined with the rate of 2.4m wii's a month all next year is + 28m.
That puts the wii at 71m without to much trouble.
Possible that in the later half of 09 the wii could be ramped up to its optimal Rate of 3m a month. Which could get the wii around 75m +.