I installed today's build for the Nano and I lost icons altogether. I disabled/re-enabled icons in the settings, tried the 'rockbox_default_icons.cfg', but no luck. There is a 'viewers.bmp' file in the /.rockbox/icons/ folder, so that can't be the problem (unless there are supposed to be other files there). I did have icons in my previous installation (2007-04-02). I guess it's just a bug, unless someone knows a reason why the icons could be invisible.
By the way, the UI appears to be broken in this build in more places. When I browse through the menu, the screen often forgets to update, e.g. when changing a setting or going up in the menu, I have to scroll or press a button to force a screen refresh...