As summary says. It's often used by USE=doc to build documentation, e.g. Portage itself is using it, dev-util/comparator, app-misc/lcdproc, and so on Please make sure you have up to tree CVS checkout since e.g. zmedico just (today) added a workaround to stable portage's ebuild and I've committed a last minute change to comparator
Stable for HPPA.
x86 stable
+ 11 Jul 2009; <chainsaw@gentoo.org> xmlto-0.0.22.ebuild: + Marked stable on AMD64 for bug #276677; tested on a dual Opteron 2354.
Stable on alpha.
sparc stable
arm/ia64/m68k/s390/sh/sparc stable
ppc stable
ppc64 done