The .desktop file shipped with net-www/nvu ebuild has a typo in it. It search for the icon as /usr/lib/nvu/icons/mozicon16.xpm , but it is installed in /usr/lib/Nvu (with big N). The attached file fixes the problem (is it really needed? a sed over the file is probably quicker :) ).
Created attachment 46410 [details] nvu.desktop
Just fixed that. Sorry about that :P.