Description of enhancement: Due to the recent mirrorlist problems I think a fallback routine should be executed by yum when the mirrorlist in unable to be retrieved that utilizes a locally stored copy of the mirrorlist. YUM would always download and retain a copy of the mirrorlist when it is avaliable, but when it is not, it would use a locally maintained file containing the contents of the last downloaded mirrorlist.
This has way too high of a chance of stale mirrors being given, though, especially as we make changes to the mirror infrastructure to only give out synced mirrors.