An estimated 6.8m smartphones were activated on Christmas Day 2011

"[...] because of its insight into application usage, Flurry is also able to see the number of new devices activated. Phones and tablets are always a hot Christmas item and this year was no exception.
On the average day in December, 1.5 million phones were activated, but on Christmas, 6.8 million were activated, representing a 353 percent spike. Last year, Christmas held the previous single-day record with 2.8 million device activations."

Source:  Data from Flurry Analytics, reported by All Things D, 27th December 2011
Update - Engadget estimates the breakdown between iOS and Android