Comment 6 for bug 633596

I take that back. I have no idea why that worked the last time. It seems similar to this bug which has persisted for several months: https://bugs.launchpad.net/xserver-xorg-video-intel/+bug/554569

I found (as they did) that connecting an external monitor to the laptop "helped" it boot. I also found that lowering the brightness completely makes the system unstable.

I'm using the workaround suggested there (xforcevesa i915.modeset=0) until these bugs are worked out.