The new Simple Locker uses Ariya, the former default wallpaper, as background.
Version: 4.10 RC1
Git commit ad5c3c7890f90b7165b5eaaaff78e79eeb884e01 by Marco Martin.
Committed on 24/12/2012 at 11:54.
Pushed by mart into branch 'KDE/4.10'.
new default wallpaper name is Elarun
M +1 -1 desktoptheme/air/metadata.desktop
No idea which bug number the commit fixes but the wallpaper in the Simple Lock Screen (what this bug is about) still uses Ariya in RC3.