Portage did not pull in an update to x11-drivers/xf86-video-intel when xorg 1.19 was marked stable and updated, causing X to fail to start due to ABI incompatibilities. Manually upgrading x11-drivers/xf86-video-intel fixed the problem.
Yes, the reason is that VIDEO_CARDS="i965" does not depend on xf86-video-intel any more (with >=1.19 it depends on xorg-server[glamor] and uses the modesetting driver by default). Because xf86-video-intel is not required, it is not rebuilt, but then it's still installed so without emerge --depclean the xserver will try to load it, failing to start... I'm not really sure how to handle that :( *** This bug has been marked as a duplicate of bug 600028 ***