OK, first some explanations:

systemd is a modern sysvinit & RC replacement for Linux systems. It is supported in Gentoo as an alternate init system. (Source: http://wiki.gentoo.org/wiki/Systemd)
ConsoleKit is a framework for defining and tracking users, login sessions, and seats. (Source: http://wiki.gentoo.org/wiki/ConsoleKit)
udev is the device manager for the Linux kernel. Primarily, it manages device nodes in /dev and handles all user space actions when adding/removing devices. Also have a look at eudev a gentoo fork of udev. (Source: http://wiki.gentoo.org/wiki/Udev)

At first look they don’t seem to have sth in common. These are some other facts:

systemd and udev don’t like each other
systemd conflicts with consolekit
consolekit is not longer maintained
systemd *is* udev (as systemd somehow includes udev)
polkit depends on consolekit and they will both pull eath other
consolekit has been replaced by logind which is being provided by systemd
…and top of that OpenRC is somehow also involved in this whole thing.

As you may have noticed: THIS IS A WHOLE BUNCH OF BULLSHIT GOING ON! I just wanted to update my Gentoo system and got stucked in some stupid conflicts between the mentioned packets. It took about one day to solve it (after reading lots of forum threads of people complaining about the same thing). Check:

Be sure you read them. They’ve helped me a lot to understand this whole topic. So after all this is how I was able to solve it:

Remove all USE flags related to consolekit:

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# euse -D consolekit policykit upower udisks

Make sure you have a clean /etc/make.conf. Here is mine:

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# These settings were set by the catalyst build script that automatically
# built this stage.
# Please consult /usr/share/portage/config/make.conf.example for a more
# detailed example.
CFLAGS="-O2 -pipe"
CXXFLAGS="${CFLAGS}"
# WARNING: Changing your CHOST is not something that should be done lightly.
# Please consult http://www.gentoo.org/doc/en/change-chost.xml before changing.
CHOST="x86_64-pc-linux-gnu"
# These are the USE flags that were used in addition to what is provided by the
# profile used for building.
ACCEPT_LICENSE="*"
ACCEPT_KEYWORDS="~amd64"
AUTOCLEAN="yes"
VIDEO_CARDS="intel"
USE="acpi smapi mmx sse sse2 cairo svg -kde qt4 qt3support dbus \
     declarative jpeg png vorbis ogg pulse alsa usb sdl spice gtk gtk3 \
     -gnome-shell -consolekit -policykit -upower -udisks"
MAKEOPTS="-j2"
FEATURES="-userfetch"
#CCACHE_DIR="/var/tmp/ccache"
#CCACHE_SIZE="2G"
#source /var/lib/layman/make.conf

# Set PORTDIR for backward compatibility with various tools:
#   gentoo-bashcomp - bug #478444
#   euse - bug #474574
#   euses and ufed - bug #478318
PORTDIR="/usr/portage"

Make sure you have selected the right profile

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# eselect profile list
Available profile symlink targets:
  [1]   default/linux/amd64/13.0
  [2]   default/linux/amd64/13.0/selinux
  [3]   default/linux/amd64/13.0/desktop *
  [4]   default/linux/amd64/13.0/desktop/gnome
  [5]   default/linux/amd64/13.0/desktop/kde
  [6]   default/linux/amd64/13.0/developer
  [7]   default/linux/amd64/13.0/no-multilib
  [8]   default/linux/amd64/13.0/x32
  [9]   hardened/linux/amd64
  [10]  hardened/linux/amd64/selinux
  [11]  hardened/linux/amd64/no-multilib
  [12]  hardened/linux/amd64/no-multilib/selinux
  [13]  hardened/linux/amd64/x32
  [14]  hardened/linux/uclibc/amd64

Adjust profile settings

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# cd /etc/portage/profile
# cat use.force
-consolekit
# cat use.mask
consolekit
-systemd

Remove udev, consolekit and polkit

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# emerge -aC udev consolekit polkit

Now you should be able to install systemd

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# emerge -pv systemd

Now update your system

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# emerge --keep-going --jobs 4 --update --newuse --ask --deep --with-bdeps=y @world

Additional steps to clean up your system:

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# emerge -a --depclean
# revdep-rebuild -- -q --keep-going