Michele Cerreta (Pizero) announced the milestone himself via Twitter:
The 'Burning Platform' title was a reference to Stephen Elop's famous 'leaked' memo of the same name, referring to Symbian and Meego being 'burning platforms' and recommending that the sensible thing to do was to jump off into the icy water, get rescued, dried off and then start again (with Windows Phone).
Regardless of your views on the politics of Nokia mobile OS development, the Pizero theme of the same name is undoubtedly both attractive and practical on AMOLED screens:
From our previous story on the Burning Platform theme's launch:
Homescreen and main application menu - the way the flames lick along the bottom and up the side is really rather splendid.
Pizero's usual attention to detail is here, with every element customised and themed (e.g. the current day highlight on the Calendar).
You can download the Burning Platform theme yourself for free, here in the Nokia Store.