Just a simple Splash Screen for QVGA (240 x 320 px) with the new "Windows Phone" logo.
The floral pattern in the background is the same as in my S2U2 wallpapers for HP Devices [link]
Copy the file (without changing its name) to Windows folder on your Device and overwrite the old one and Voilą - you've replaced the ugly Green Bootup Screen!
Of course you can backup the original first (I didn't 'cause I won't use it ever again).
Tested on WM6.1 - should work on WM5 and WM6 but if I were you I would double check (or double-Googled) to be sure.