As usual, I have completely forgotten any relevant system/software
info, so:
This is a Thinkpad T420 with:
00:02.0 VGA compatible controller: Intel Corporation 2nd Generation
Core
Processor Family Integrated Graphics Controller (rev 09)
I've just sent a mesa-9.1-2 build for koji for F19 that should workaround this,
we don't have a proper fix yet, but the chromeos developers had a hack in their
tree they pointed me at, and it does appear to let me boot again with RC6 enabled.
Dave.