I recently realized, that my X220 was not allowing the gpu to use RC6.

Just modify your lilo.conf and run lilo. After reboot you get a system with <8W idle power consumption :)

# LILO configuration file
# generated by 'liloconfig'
#
# Start LILO global section
# Append any additional kernel parameters:
append=" vt.default_utf8=0 i915_enable_rc6=7"
boot = /dev/sda

Slackware-Current (March 2012) works quite well. But you'll need to update XF86-Input-Wacom to get the touchscreen working.

Just upgrade the package (can be found here) and everything is fine :)