With the most recent --sync is a upgrade from cifs-utils-6.0 to 6.0-r1. And this all seems to be related to samba. You will get this message with -uavND world.

Calculating dependencies... done!

!!! The ebuild selected to satisfy "net-fs/cifs-utils" has unmet requirements.

• net-fs/cifs-utils-6.0-r1::gentoo USE="acl caps caps-ng -ads -creds -upcall"

The following REQUIRED_USE flag constraints are unsatisfied:
acl? ( upcall )

(dependency required by "net-fs/samba-3.6.13[client]" [installed])
(dependency required by "media-tv/xbmc-12.1[samba]" [installed])
(dependency required by "@selected" [set])
(dependency required by "@world" [argument])
funserver funfool #

I use xbmc and believe samba was pulled in as a dep this way. Here is more info on upcall from this link.
http://www.samba.org/samba/docs/man/manpages-3/cifs.upcall.8.html
cifs.upcall is a userspace helper program for the linux CIFS client filesystem. There are a number of activities that the kernel cannot easily do itself. This program is a callout program
that does these things for the kernel and then returns the result.

The problem I see, If I add upcall globally in make.conf, it wants to downgrade xorg-server.

These are the packages that would be merged, in order:

Calculating dependencies... done!
[ebuild N ] sys-apps/keyutils-1.5.5 62 kB
[ebuild N ] dev-libs/libev-4.15 USE="-static-libs" 502 kB
[ebuild N ] dev-libs/libverto-0.2.5 USE="libev threads -glib -libevent -static-libs -tevent" 342 kB
[ebuild N ] app-crypt/mit-krb5-1.11.2 USE="keyutils pkinit threads -doc -openldap {-test} -xinetd" 11,400 kB
[ebuild N ] virtual/krb5-0 0 kB
[ebuild U ] net-fs/cifs-utils-6.0-r1 [6.0] USE="acl%* caps caps-ng* upcall* -ads -creds" 0 kB
[ebuild r UD ] x11-base/xorg-server-1.12.4-r1:0/1.12.4 [1.13.3:0/1.13.3] USE="ipv6 nptl udev xorg -dmx -doc -kdrive -minimal (-selinux) -static-libs -tslib -xnest -xvfb (-suid%*)" 5,318 kB
[ebuild rR ] x11-drivers/xf86-input-keyboard-1.7.0 0 kB
[ebuild rR ] x11-drivers/xf86-input-evdev-2.8.0 0 kB
[ebuild rR ] x11-drivers/xf86-input-mouse-1.9.0 0 kB
[ebuild rR ] x11-drivers/xf86-video-nv-2.1.20-r1 0 kB
[ebuild rR ] x11-drivers/xf86-video-vesa-2.3.2-r1 0 kB
[ebuild U ] kde-base/kwin-4.10.2-r1:4 [4.10.2:4] USE="opengl (-aqua) -debug -gles" 0 kB

Total: 13 packages (2 upgrades, 1 downgrade, 5 new, 5 reinstalls), Size of downloads: 17,623 kB