Last night, I hammered out the most important part of implementing layers -- with latest CVS, you can use the XML editor to set the inkscape:groupmode attribute of a group to "layer", and be able to edit its children directly.

There's still a lot of UI and refinement work to do, however; I've outlined it on the Wiki here:

http://www.inkscape.org/cgi-bin/wiki.pl?DocumentLayers

-mental