I actually liked this idea, as I've always been slightly surprised by the initial result whenever I start drawing something in Inkscape, and this would make that absolutely clear. So I've made an initial patch that implements this (if I missed a tool, it's easy to add one, just needs a small change to TOOL-context.cpp and some recoloring of cursor-TOOL.xpm).
I'm interested in people's experiences, so I've attached a patch and uploaded a Windows build to: http://home.hccnet.nl/th.v.d.gronde/inkscape-color-cursor.zip (just inkscape.exe, so backup your current inkscape.exe, extract the zip and your good to go)
One of the main glitches it currently has is that the "preview" doesn't change if you select a different color (you have to reselect the tool). Any ideas on fixing that would be appreciated (just reply to the devel list only).
Wow! So fast. Magic! :) I would love to hear feedback, too. Especially, since I did not test it (mac...)
Alex