Ballmer's making some pretty ambitious projections there.
Windows 7 sold 240 million copies in its first year. Assuming Windows 8's UI design doesn't hurt adoption by the traditional desktop and laptop crowd, lets give that same number to W8.
If Windows Phone 8 holds to WP7's results for last quarter, they should expect over 20 million sold over the year.
That leaves almost 140 million new W8 buyers, presumably via new tablet/touch oriented hardware. That's over 1.6x the number of total iPads sold as of last March. Either that or WP sales have to rise dramatically to help close the gap.