Those messages are most often created when an update has been made to the primary repository but the mirror repositories are not yet in sync. (The checksums are updated first since they're small files, then the big repository update is done. It's done that way so you won't download an obsolete release.)
After few hours the repository updates are, usually, finished. (It obviously depends on the mirror repositories access speed, but mot mirrors are on T1 or T3 connections, so even a multi-gigabyte update is usually finished fairly rapidly.)
So, try again later.