Ballmer says during Christmas, Windows Phone was selling five times more devices than the same period last year.
(via The Verge, and others).
Assuming that there isn't a rounding error in here that is being played on, that's an increased rate from the launch period, where Ballmer leaned on a 'four times' multipler on sales from the year before.
Taking the number at face value, let us do some number crunching on sales and make a reasonable estimate at a number. Our data points are last year's sales figures, and Ballmer's multipler. Let's go for the lowest it could be, 4.5 times (and Ballmer rounded up). Gartner last year reported sales of 2,759,000 Windows Phone handsets during Q4 while Canalys reported 2,500,000 handsets. Going for the lower number again with the multipler and we get 11,250,000 Windows Phones.
Hold on a minute though, Ballmer said Christmas, not Q4. The Christmas in my mind is the four weeks between Thanksgiving and December (i.e. the last pay cheque before the holidays). I'll let WP have 4.5 for four weeks, but 3.5 times for the previous nine weeks (i.e. the lowest number that can be rounded up to Ballmer's four times).
That gives us 9,519,230 - give or take, that's a conservative nine and a half million. If you take the more generous numbers and the upper estimates and rounding errors, you can estimate sales of 13,263,281.
I'd guess that most of the rounding to get to the 'four times' and 'five times' phrases means the numbers were just underneath the cardinal numbers (otherwise we'd have 'more than five times' in the phrasing) so the lower end of our estimate is more likely. Even then, that should give Windows Phone Q4 sales of over ten million units.
How many of them are of the new Windows Phone 8 handsets, how many are mid-range Windows Phone 7.5 devices such as the Lumia 710 and 800, and how many of them are new customers as opposed to people upgrading to a new WP handset... all that remains to be seen.
But if Windows Phone has hit ten million devices, then that's another small milestone to celebrate.