I enabled a pre-commit check which validates if there is a /trunk/MAINTAINERS file. It also quickly checks if the format of that file is ok. If a module doesn’t have a /trunk/MAINTAINERS file, or it isn’t in the right format, the commit will be rejected. See http://live.gnome.org/MaintainersCorner#maintainers for how a proper /trunk/MAINTAINERS file should look like.
Note that I gave various pre-warnings regarding this, and your commit is not lost. You can commit a proper /trunk/MAINTAINERS file to allow normal commits again.
Since enabling the pre-commit just over an hour ago, 6 additional modules have a proper /trunk/MAINTAINERS file. Only a few hundred to go.