XCache 3.0.1 has a proven nasty segfault with php fpm that has just fixed in svn. Please see: http://xcache.lighttpd.net/ticket/301#comment:6
We currently have quite a bit to do these days, so maintaining snapshot ebuilds for something like xcache is not a priority. Will bump once xcache 3.0.2 is out, otherwise there are other alternatives for opcode caches.
Confirmed on multiple servers; segfault when threads are used. XCache 3.0.3 fixes this and is out from 06/18/2013, please bring it into portage/Gentoo, thank you.
v3.0.3 has been in the tree a while