When I do yum upgrade from fc6 => fc7 (yes, I know this isn't supported) I get the error message: Package libwnck-2.18.2-1.fc7.i386.rpm is not signed Doing a yum remove libwnck gets rid of the problem (I don't use this package, or others dependent on it, anyway). But still, this indicates a bug, no?
This was fixed shortly after the unsigned package was noticed.