It's going to be difficult for Wii to accoplish this, as 360 will falter but PS3 will likely take up those sales. Wii will probably end up at about 40-45 million in sales by the end of 2008. 360 will likely be around 24 million and PS3 around 20 million. So Wii would definetly have to hit that max of 45 million, which is unlikely, but possible. But Wii in its lifetime may never be able to get 50%, unless of course PS3 doesn't take over 360's increases in America, as the 360 falters.
Nintendo will not be seeing 45 million in sales (or even over 41 million) because they can't sell inventory that won't exist.
Once again: 19.153 million units from initial release through Dec 30th 2007.
Add 1.8 million x 12 = 21.6 million sold for the year of 2008 for a total of 40.753 million by Dec 30th 2008.
That number is set in stone contingent upon two simple projections.
1) Wii sells out through 2008.
2) Nintendo changes its policy on production (1.8 million/month).
"Ultimately, Wii production numbers -- and the United States' allocation of consoles -- are determined by Nintendo's home office in Kyoto, Japan. Harrison says the company will continue producing 1.8 million Wiis every month until demand subsides."