emerge ibutils fails because the downloaded source package was repackaged with a modified extension. another user on #gentoo mentioned the following in IRC chat: Indeed, the ebuild for sys-infiniband/libibverbs-1.1.4-r1 has OFED_SNAPSHOT="1" defined. So it seems like a similar line is needed for the ibutils ebuild.
+ 24 Apr 2012; Kacper Kowalik <xarthisius@gentoo.org> ibutils-1.5.7-r1.ebuild: + Define OFED_SNAPSHOT wrt #413155 by <jarray52@gmail.com>