At last system update xorg-server was switched from 1.3 to 1.5. The update was finished normally, revdep-rebuild find nothing. But the restart X-server failed. The main reason was: (II) Loading /usr/lib/xorg/modules/drivers//sis_drv.so (II) Module sis: vendor="X.Org Foundation" compiled for 1.3.0, module version = 0.10.0 Module class: X.Org Video Driver ABI class: X.Org Video Driver, version 1.2 (EE) module ABI major version (1) doesn't match the server's version (4) (II) UnloadModule: "sis" (II) Unloading /usr/lib/xorg/modules/drivers//sis_drv.so (EE) Failed to load module "sis" (module requirement mismatch, 0) so, I had to manually re-emerge x11-drivers/xf86-video-sis to start xorg-server.
xorg-server post-install note explicitly tells you to do that. I think it has to be that way, so xorg-server upgrade can force driver upgrade (if it's necessary).
The x11-base/xorg-server-1.5* ebuilds explicitly say: You must rebuild all drivers if upgrading from xorg-server 1.4.1 or earlier, because the ABI changed. If you cannot start X because of module version mismatch errors, this is your problem. You can generate a list of all installed packages in the x11-drivers category using this command: emerge portage-utils; qlist -I -C x11-drivers/
