The way i solved this issue was to create a dummy package for flashplugin-nonfree
edit the created file 'flashplugin-nonfree' and change line 8 from
sudo apt-get install equivs
Package: <package name; defaults to equivs-dummy>
save the file
install the resulting .deb file
now you can install boxee with no complaints....
dpkg -i flashplugin-nonfree_1.0_all.deb
if you do need to add the true flashplugin-nonfree package later, remove the dummy package.