The Fedora Project recently re-signed all of its packages with a new key. The story regarding the key can be found here. Contact the release engineering team via IRC in #fedora-devel (irc.freenode.org) or via email to email@example.com for more information.
Users need to accept the new signing key the first time updates are downloaded. PackageKit prompts you whether or not to import the signing key. To accept the key, press y at the prompt.
After the new packages have been released for a period of time (not yet determined), the Fedora Project will release a new package to forcibly remove the old key from the system's package database (or rpmdb). This method ensures the old key is no longer trusted/used by the system.
The plan for implementing the new key is still fluid, and therefore may change. Updates to this page will occur as new information becomes available.