The new Simple Locker uses Ariya, the former default wallpaper, as background. Reproducible: Always
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 http://commits.kde.org/kde-runtime/ad5c3c7890f90b7165b5eaaaff78e79eeb884e01
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.