As per summary; when using v8, it has to be pax-marked as it uses RWX mappings.
Thanks Diego, finally took the time to fix this : + 19 Mar 2012; Ultrabug <ultrabug@gentoo.org> mongodb-1.8.4-r1.ebuild, + mongodb-2.0.2.ebuild, mongodb-2.0.3.ebuild: + PaX mark mongo/mongod for v8 installations, fix #399605. +