A comparison between dark GTK themes with both sets.
http://www.ohweb.com.ar/screenshots/Inkscape/Tango-Icons-Colors-dark.png
Great comparison. If nothing else I think this points to a need to at least fix the current icons to handle dark backgrounds more gracefully.
One question -- with the pen/pencil/calligraphy tools now looking so similar to the original versions, why not just use the more distinctive originals with just the black lines modified to work on dark backgrounds? The images of the tools just pop out for me a lot more, and look more realistic in the current inkscape set. Especially the fountain pen.
And thanks for making some of the changes I suggested, like to the gradient tool. These icons don't have a chance without someone taking an active role, like you are, to incorporate as much community feedback as possible.
--bill