http://www.gentoo.org/proj/en/gentoo-alt/prefix/techdocs.xml#doc_chap2 (referenced for ED and EROOT info from the devmanual) says that ${ED} is ${D}${EPREFIX}/ ${EROOT} is ${ROOT}${EPREFIX} but PMS (which portage agrees with) says ${ED} is ${D%/}${EPREFIX}/ ${EROOT} is ${ROOT%/}${EPREFIX}/ While these refer to the same directories, the distinction matters when people try to manipulate paths as strings (e.g. in bug #434956, where code is expecting ${EROOT#${ROOT}} to give ${EPREFIX}).
fix (and update) committed, thanks