Each cycle Matt Domsch runs a mass rebuild against all fedora packages:
http://fedoraproject.org/wiki/FTBFS

Those that fail to build from source have bugs filed on them to be fixed.

After the Alpha we should orphan all packages that have FTBFS bugs that are in "NEW" state (ie, not looked at by the maintainer). This would give other folks a chance to pick up the package and fix it so it builds. In the event that the package is not taken over or fixed, it would be dropped at the same time as all the other orphans are dropped.

Timing for this should be determined and added to the rel-eng calendar and list of tasks as well as a SOP written up to handle how to do this.