all boson ebuild on the portage tree are broken. I'm trying to put the right ones for some weeks now you can find the latest one, tested, on http://orzel.freehackers.org/download/kvim-6.1.141_rc1.ebuild.tgz it will create app-games/boson. (that's where the broken ebuilds are already)
argh.. the file is at : http://orzel.freehackers.org/download/boson-0.6.1-r2.ebuild.tgz bad copy-past, sorry...