Closest I can see is:
Code:
rootCommand:			bsetroot -gradient flatcrossdiagonalgradient -from rgb:4/6/8 -to rgb:3/48/6
in /usr/share/fluxbox/styles/Operation
which is referenced in ~/.fluxbox/init

Operation being the style I'm using.

My guess it;s something nasty that KDM is doing, as it's the kdm background. I might have a go at editing the KDM files.
Currently, running restart from the fluxbox menu resets the background to what I want......

Hrm.
--
TimW