Important PSA for Gentoo users:
If portage updates you to x11-base/xorg-server-1.10.2, YOU MUST MANUALLY RE-EMERGE ALL OF YOUR INSTALLED X11 DRIVERS. Portage will not do it for you automatically because it does not know that the ABI has changed. If you do not do this, X will fail to start.
Here's a one-liner to do the job:
emerge -1 $(eix -I x11-drivers/* | grep x11-drivers | awk '{print $2}')
(For clarification, there's nothing new about major version bumps in X.org changing the ABI. On most previous cases, though, such as when 1.7 and 1.9 came out, the postmerge notes on the ebuild either noted that the ABI had changed and all drivers needed to be rebuilt, or pointed to the postmerge notes on the main X.org site, which told you the same thing. This time, however, this advisory was omitted for some reason. So, if you don't happen to remember that the version you're updating from is NOT a 1.10.x subversion, you can find yourself with an X that won't start.)