From a Microsoft press release:

Microsoft has now sold 55 million Xbox 360 consoles across 38 countries and sales are accelerating. Xbox 360 maintains its lead in the U.S. as the top selling console in 2011, and ten of the past eleven months. In the first four months of 2011, Xbox 360 has seen a 29 percent year over year increase in sales, which is more than triple the growth of PlayStation 3 and the largest growth of any console on the market.

That is legitimately impressive, even after you account for a few million units bought to replace old, broken hardware. There's life in the old girl yet.