Unzip and place this UINavigationBarShadow@2x.png into the /your.theme/UIImages folder to remove the thin grey bar on the lockscreen if you've hidden the slider. (see http://i.imgur.com/8iSXCsf.jpg)

The downside is it removes shadows from *all* navigation bars (such as under address bar in safari) so everything else looks a bit "flatter".

There is also a shared@2x.artwork for (iPhone 4 / 6.1) non-winterboard themers at http://dl.dropbox.com/u/38330/Shared%402x.artwork

-dirtysnachez @ /r/jailbreak